I don't think ARC2 can help me much, since by necessity and preference I mix with headphones. 
 
I'm looking for a very basic fix: increasing the amount of treble in the mix downs. 
 
Is an EQ on the master audio track the best way to go?

I wouldn't. It's too blunt an instrument
 
If you HAVE to do this I'd use the built in Quad Curve EQ on each track that needs it.
 
You'll find that not every track does need it, certainly not kick & bass!

Sorry - The phones part didn't register.
 
Sonarworks and headphones to match or A/B with reference tracks.  Or both :)  I would go with Jonesey's suggestion though as far as where to do the EQing.
 

Tripecac
I don't think ARC2 can help me much, since by necessity and preference I mix with headphones. 
 
I'm looking for a very basic fix: increasing the amount of treble in the mix downs. 
 
Is an EQ on the master audio track the best way to go?

Yes and NO... it all depends... what ur essentially doing is compensating (blindly) for what ur headphones AREN'T translating to ur ears when u mix.
 
I've visited this thread a few times (It's LONG and OLD!) and am NOT going to read it again to see if you took the suggestion of MANY of us to use Sonarworks... It gives u a FREE F'n 30 day trial... and IF you don't have one of their certified headphones, GET THEM and TRY remixing in Sonarworks...
 
I ALWAYS was frustrated with my mixes.  I got the KRK's because they were widely hearlded as among the flattest of HPs... Still, till I tried Sonarworks, my HP mixes didn't translate well... NOW THEY DO!!!!   Someday I'll find the time to remix all the old stuff... there is NO comparison.
 
Seems to me you're just wallowing in circles.  No offense, and I apologize if you are now using SonarWorks+certified Headphones and still have the problem... 
 
If u aren't, quite blathering and trial SonarWorks... and go down to Guitar Center and get one of their high end certified headphones like the KRKs  You can ALWAYS return them if this doesn't work.  But I know of NO ONE who hasn't found this to be a HUGE mix improvement.
 
BTW, equing the final mix could have been tried in the time it took to write this.  If that works, great... but it's no substitute for tuning ur HPs, if that's what ur gonna use.
 
Personally I go back and forth btw monitors and hp (Both KrKs) and actually used sonarworks to turn up/down my subwoofer so it works in the room.  Bottom line, Sonar"WORKS:!!!

Tripecac
Thanks a bunch, guys!  It sounds like the best way to get good mixes is to mix on good speakers.  Like fireberd, I have some Samson Resolv 6.5a monitors, but they've been sitting in their box for about 5 years now, mostly because we've been renting houses with thin walls and no sense of privacy (having 2 small kids doesn't help since they'd bang on the door the instant they hear sound emerging from my office).  Perhaps when we get our own house, and the kids get older, I'll be able to break the dependency on headphones but until then I'm afraid I'm stuck.
 

Hello ,
had for several years a compareable situation and give the monitors a try -
and to my surprise I managed much better mixes at  low loudness and the kid adapted 
hearing music out of my office.
I learned that good studio monitors work at low loudness.
 
Why not giving it a try ?
regards
 

I'm a clumsy amateur with limited budget and skills, but anyways..After learning my equipment I mainly trust two things: Reference track and SPAN.
 
If the project sounds decent compared to the reference track and looks decent in SPAN (compared to the ref. track curve), it usually is about as good as I'm able to achieve. It doesn't mean it's soundwise equally good, but the frequency balance is ok.
I use both monitors (Behringer 2031) and two pairs of headphones (nice Sony Hifi, don't remember the model, and DT 770).  Using the DT 770, which are a bit bass heavy, I let the mix sound heavyish, using monitors I let it sound slightly thin.
When I finally compare the curves in SPAN, I'm usually very close to what can be accepted both by my ears and eyes.
 
For example: If the high end sounds very bright and airy with the DT 770's (esp. acoustic guitars á la Tom Petty and the likes) they are intolerably crispy through the hifi headphones but tolerable through the Behringers. So, the high end gate keeper is mainly the Hifi headphones/SPAN, and for the low end all of them, SPAN actually saying the last word.
Then, when I take it out to other systems, it's usually more or less acceptable. Sounds like hi-hat and cymbals being usually the ones that need further tweaking most. What I find most problematic at the moment is creating bass tracks that have enough harmonics and whatever to sound proper on small systems that lack all real bass. I like the solid, round, beefy sound with little distortion, but it vanishes on minisystems.

Anderton

jpetersen
 
For this reason I start off mixes in mono so the instruments will "collide" with each other as much as possible. Then, you can adjust EQ to emphasize or de-emphasize particular instruments. Once everything sounds distinct and has its own space in mono, when you start creating a stereo field the whole mix widens up and creates a full, well-defined space. It's also more translatable, because the playback system's frequency response becomes less crucial as you've already done frequency response tailoring. You're not counting on the playback system to be capable of nuanced response.

This is actually something I do quite often, Ill build an entire session in mono.  It helps me avoid trying to be to clever in my mix when I should be trying to keep it simple.   I would spend all this time doing super tricky stuff, manipulating the stereo field with all of this trickery... get to the end of the mix, and then finally click that mono button... and it sounds like feeding time at the zoo.... (cell phones.. for example... a huge source of youtube and soundcloud plays.. are in mono...)..  A huge source of plays are youtube and facebook video... soundcloud.. etc..  people will hear it on the speaker of their cellphone which is ... mono... (unless they use headphones)
 
As far as low end, Ill high pass percussion like tamborine..shaker etc.. but a traditional high pass will potentially create a peak above where it is passed... so if you use it to make something less "boomy" - all of a sudden it might become a little "muddy" (150-300ish).. or "cardboardy" (400-700ish).  Some eqs.. like the pultec style, use this peak and dip in a super musical way more than others. 
 
I used to high pass everything... and it has a sound for certain.   
 
One of my preamps is a black lion b173.. a neve clone.  It gets its sound by creating all of this mild overdrive in the low end, and makes everything it touches much bigger and fatter as a result.  However - if you looked at the rta you would see all of  this low end noise.  I started getting great rock guitar sounds when I began using it... so here I learned that when I high pass rock guitars.. often they sound wimpy..
 
Use the sonar eq - it has a great default rta on each channel and see what is actually happening down there.  
 
Use a reference track or 2 right in the session.. always go back to it ..especially if you use headphones.. you will more than likely find that you are "Overmixing" based on your memory of what something sounded like, verses an at your fingertips immediate sonic comparison.   You might remember the bass from a song at a club... or the guitar from the song at the supermarket..etc...  you need to hear what inspiration you are drawing from right there in your mix... in your session.  Also - you'll spend 99 cents and support another artist just like you~

Guys, I don't have the ears, experience, time, or budget that you guys have. 
 
Although I've been making music since the '80s, I don't make any money from it, and, now that I have 2 jobs and 2 kids, I have barely enough time to squeeze out a CD's worth of music each year.  I live in a small NZ town, so there is no "guitar center" down the road as Vastman implies (not every town is an extension of the Great American Suburb, you know).  There might be some other DAW musicians in town, but I don't know them, and, if they are anything like me, they are probably not going to announce themselves to me as such as I jog on the forest trails or bike along the lake with my girls.
 
Sonarworks costs hundreds of NZ dollars just for the headphone calibration.  Getting new headphones on top of that is just way outside of my music budget.  Plus, even if someone bought Sonarworks for me (care to volunteer, Vastpockets?), I would have no idea how to use it, or how to trust my ears.  What sounds "right"?  I have no idea.  No clue. 
 
All I know is that when I listen to my mixes on anything other than my Sennheiser HD 580s, the treble is lacking. 
 
Simple solution: stop mixing on the HD 580s.  But my only other good headphones are my HD 590s, which I use all day for listening to music.  If I had a single sound card in my PC, things would be easy.  But that thread kinda fizzled: http://forum.cakewalk.com/Sound-card-to-replace-Delta44-and-Xonar-m3464445.aspx  and plus, another sound card, means more $$$.
 
So, I'm after a simple, convenient, FREE solution, and to me, that means looking for a single "button to push" which will fix the mixes. 
 
The solution I proposed is a treble-accentuating EQ effect on my master track in my default template.  It would start of disabled, and then I'd turn it on right before mixdown (after I have completed the mixing).  Voila, treble.  I tried it yesterday, but haven't had time to listen to the mix on the main stereo.
 
It seems like a simple fix, but what I'd like to know is if there is a better (simple, tree) way to boost the treble post-mixing? 
 
For example, is there a mastering effect that will automatically detect that my mixes have too little treble, and will automatically boost the frequencies which are lacking?
 
Or, does it make more sense to deliberately enable a "muffling" (treble reducing) EQ effect during mixing, and then remove it for the mixdown?  This would force me to add more treble during my mixes, and *might* make the final balance better in the long run, but I have no idea!
 
And finally, if I can fix the treble for the songs on my current album, I can use the same technique on my previous albums, so we're talking hundreds of songs...  This is why I want a simple fix.

Sometimes the best way to boost the treble is to cut the bass. That's what a simple tone control does. I think your solution of finding an EQ set up for your final export will work well and cost nothing. Experimenting will cost time but no hard cash outlay. There are also EQ options in Cakes export program where you can manipulate the EQ.

Simple solution: stop mixing on the HD 580s.  But my only other good headphones are my HD 590s, which I use all day for listening to music.  If I had a single sound card in my PC, things would be easy.  But that thread kinda fizzled: [link=http://forum.cakewalk.com/Sound-card-to-replace-Delta44-and-Xonar-m3464445.aspx]http://forum.cakewalk.com...nd-Xonar-m3464445.aspx[/link]  and plus, another sound card, means more $$$.

 
Well your answer is staring you in the face.
If you listen to music on your 590's all day, then you know intuitively what a good mix & frequency response sounds like. I own a pair of 280's and whilst their isolation is GREAT for tracking, I would never dream of mixing on them
 

So, I'm after a simple, convenient, FREE solution, and to me, that means looking for a single "button to push" which will fix the mixes. 

 
Sorry, but there is no magic bullet in any aspect of this business. Just remember that ALL monitoring systems will lie to you about your room and therefore your mixes, but headphones tell a MUCH bigger lie.
Save up for some monitors