When I first started mixing in earnest, I was using a small Yamaha PA for monitoring. I thought it was a pretty decent little PA. At least, the records I played through it sounded OK, and my mixes were sounding pretty good through it. Boy, was I in for a shock the first time I burned a CD and took it out to the car! The bass was rattling the windows, but only at certain (and particularly annoying) frequencies.

Then I took the same CD down to my recroom and played it through the stereo I use with the TV. This system ain't hi-fi, but it's got a powerful amp and big speakers (2 18" JBL subs, crossed over at 120Hz + Altec Lansing 10" 3-way cabinets). To my shock and puzzlement, the CD sounded thin and bass-light! WTF? The same mix sounded awful on two different systems, but awful in completely different ways! At that moment, I really wondered if translatable mixes were ever going to be a possibility for me.

That was the start of a long journey involving book-larnin', experimentation and laying out hard-earned dollars. Better monitors was the first step, and that helped a lot. But the big breakthrough came when I bought (and read cover-to-cover three times!) The Master Handbook of Acoustics, by F. Alton Everest. That was my doorway into a wonderfully mysterious world, and the first of what would grow into an entire bookshelf of acoustics textbooks. And ultimately, it finally led to the elusive goal (about 4 years later!) of a CD that actually sounded good in the car.

Thanks for all the very informative feedback.

iPhone's iPod has one global EQ setting, and you have to choose a preset ... this was (don't know how1) set to "Bass Booster" and I changed it to "Flat".   The car, which has a simialr rudimentary EQ setting in that you have to choose one preset (which you cannot control) - it was set to "Rock" and I changed it to "Off".    Played again under these settnigs, it did sound LESS boomy, but still, I think heavy on the bass.

I took some advice from a PM I received (thanks, Dan) and compared the EQ curve of my track against a similar professional piece.   I actually used Ozone for this - I just recently purchased, so not so sure if I'm using it right, but I took a snapshot of my piece and the professional piece, and the EQ curves of both are displayed in different colours on teh screen.    I tried this with the free Voxengo Span, but its hard to compare on 2 seperate graphs (if I'm right, its not possible to have 2 EQ curves on the screen overlaying each other).  Also the snapshot made it easier to see at-a-glance what the differences were, rather than a moving curve ... that is, if I used it right .-)).   I found, surprisingly, my EQ curve and the professional EQ curve were remarkably similar - the pro curve had more bass, actually, but only in the below 50Hz region - I'd rolled off my bass from 40 Hz, though it didn't seem like the pro one had done.   Mine was only noticeably higher, by around 1.5 db, in the 120/130 Hz region.   Not sure if this is significant or not - I pulled mine down a notch here, though I didn't really hear a change overall.   

Main thing is though, even with the car and phone EQ setting adjustments, I still find my sound a bit too bassy, though its fairly similar to a similar-ish style professional track.   Perhaps I'm just not hearing it right now, maybe I've focused to hard too long on the bass sound ... 

You can hear my track here, I'd be interested to know if it translates at all ... http://soundcloud.com/mosaic-productions/bass-testing

Thanks

Not to step on Razorwit, bitflipper or Kalle's points (or anyone else for that matter) as I feel they are right on, but I think it may be dismissive to not consider good studio reference headphones as a start. Even great engineers will swap from near-field to larger monitors and headphones to mix and master. While I would not suggest that headphones can ever take the place of good monitors one has to consider a few things.
 
Ultimately it is good to have a set of studio reference headphones no matter how good your monitor system is. Room treatment is not cheap. Having adequate cables and a good enough power amp to drive the monitors (if they are passive which I feel is often a better way to go depending on your situation/needs) is not cheap either. Tuning a room properly can be a time consuming and expensive proposition and if you do not own a space and you rent it can be problematic. Yes, you can make a rented space sound better but depending on the space it can be difficult to get marginal results (hard floors, windows, neighbours, dimension/shape etc.) and can of itself become a bit of a money pit.
 
Again it is not my intent to discourage monitors, I highly encourage them, but an entry level idea is to get some good studio reference headphones. I use Tannoy and customized ROR monitors but always turn to my AKG K240DF headphones as another check for my mixes. And they also come in very handy for tracking meaning less to fix in the mix later. Recording guitar, vocals, anything that requires mic'ing requires headphones anyways as near-field monitors will bleed into the mic.
 
EDIT: One other consideration I should have mentioned is that some headphones such as the AKG's I listed operate at different impedence. The AKG's, for example, are rated at 600 ohm which means some typical headphone jacks like an iPod will not adequately "drive" the speakers. Typically the headphone jacks of most pro-audio gear will drive them adequately, such as a mixing board. Just something to consider if phones seem like an option.

I have some old ... but I love them ... headphones : Sony MDR-7506.    I did mixes using just these - but what sounds great on just these, really didn't sound good on monitors at all.    Now I'm using Yamaha HS80s as nearfield monitors - I don't really know how good they are to be honest.   They can sound a bit harsh, particularly after listening on headphones, but I guess that's becuase the headphones sound so "good", though not flat ... 

... to be honest though, I like listening via headphones, if there was a different pair I could use that would give a more real flat response.  Ideally, track and mix on headphones, and check with monitors ... but I guess that's not the correct way ;-)

ULTRABRA


I have some old ... but I love them ... headphones : Sony MDR-7506.    I did mixes using just these - but what sounds great on just these, really didn't sound good on monitors at all.    Now I'm using Yamaha HS80s as nearfield monitors - I don't really know how good they are to be honest.   They can sound a bit harsh, particularly after listening on headphones, but I guess that's becuase the headphones sound so "good", though not flat ... 

... to be honest though, I like listening via headphones, if there was a different pair I could use that would give a more real flat response.  Ideally, track and mix on headphones, and check with monitors ... but I guess that's not the correct way ;-)

The Sony's are considered good for tracking and probably good for most general studio/listening situations, possibly even secondary monitoring (testing a mix) - comfortable, reasonably loud, good clarity. But no, I don't think they will compare to a good set of reference headphones.
 
Right now the only "correct" way is what works for you and what fits in your budget. Short term and long term goals are not always the same... :-)
 
Clearly there are choices but as I said some good reference headphones will never go to waste. Just listening to music on the AKG's is an absolute pleasure. When you get used to them and adjust to just how flat they sound and the level of detail they reveal the Sony's will no longer make you so happy. Hmmm. Maybe that's a bad thing?
 
Just out of curiosity, do you also compare your mixes to reference tracks thru your Yamaha's? Probably a dumb question but I had to ask. It's good to listen to a lot of music thru the monitors/headphones, not just your mixes. I doubt I'm telling you anything new.
 
One problem with the Yamaha's is that they are known for a slightly weak bass that rolls off rather high (as many small monitors also do) so many advise using a sub as well. That being said adjusting the 120/130 range is crucial in terms of sounding boomy or muddy (40 is getting into rumble territory) and, in my opinion, room treatment or not, adjusting this range is no small task on smaller near-field monitors. Headphones, on the other hand...
 
But who knows? Maybe incoporating a sub would find you pushing less bass in the mix and that itself could have a profound effect. Not likely it would be flat though, just another compromise.
 
The AKG's are rated to 15Hz for what it's worth as an example, but more importantly is that they are substantially flatter in the low end than the Yamaha monitors. The Yamaha's give good clarity but not the best frequency response across the spectrum which is the issue you have at the moment.
 
Too bad it isn't easier to rent this stuff short term to try it out. If you can find the right store, willing to work with you, you may be able to do exchanges within a certain period and lose nothing.

Re checking other mixes through my monitors - I do, but probably not enough.   I've started to do it more, so I can get used to pro mixes through my monitors rather than the CD player.   

I checked out your headphones - they are now "discontinued"  - do you know what would be the new version of these?     K 240 MK II I guess ...?

ULTRABRA


Re checking other mixes through my monitors - I do, but probably not enough.   I've started to do it more, so I can get used to pro mixes through my monitors rather than the CD player.   

I checked out your headphones - they are now "discontinued"  - do you know what would be the new version of these?     K 240 MK II I guess ...?

That's a tough one. I just looked at what's available and I don't see a direct comparison. The original 240 and 240M, if I'm not mistaken, were not as expensive nor as good for mixing as the 240DF. The 240MkII look similiar and being in the 240 line that's what I expect but the K702's description seems closer to the original DF's as they were touted to have a very flat response. I believe the 240DF's were about $299 retail at the time but that was about 10 years ago and even then I can't swear to that (I got my first set at cost when I worked at the Guitar Center in Hollywood).
 
It's been a while since I seriously compared any but I will ask around to see if I can get a recommendation. Of course it's you who has to make the choice and who knows, you may prefer another brand or just to go in a different direction altogether. The great thing about the Guitar Center was that it had a huge inventory and I used to have hours at a time on slow days to sit in their studio room and try out monitors and headphones. You could set up 3 sets on monitors at a time and just flick a switch to A/B them. I was spoiled. Too bad all the gear is completely different now ;-)

even with all the best gear and a well tuned room, referencing your mixes out of the studio is a good idea- even if just to gain new perspective.  
To your original question- it is likely that itunes is automatically turning your wave into an m4a file.  While this does not sound as good as a wav, it is unlikely that it is causing a bass boost.    Most of the quality losses will be in the high frequencies (noticeable in cymbals and whatnot.

At risk of sounding like a broken record, I'll reiterate the best advice I ever got: spend as much time as you can listening to your favorite commercial recordings in your room on your monitors. 

A strange thing happens in the process, which is that your ears and brain will gradually train themselves to ignore the acoustical anomalies and speaker limitations and build a mental image of what the music should sound like in your room. This epiphany came to me through another highly-recommended book, "Sound Reproduction" by Floyd Toole.

BTW, a good pair of headphones is helpful, too. It's one more reference to compare to, and eliminates the effects of room resonances. Just don't expect them to provide the last word, unless you're willing to distribute a pair of identical headphones to everyone who will ever listen to your music. The AudioTechnica ATH-M50's offer an excellent balance between cost and truth.