I just got the ATH-M50x headphones.  These were at Sam Ash, where they had a nice display where you could listen to a dozen different headphones.  I did some blind testing with a friend.  As an aside, some of the best sounding headphones, IMHO, were Samsons in the $60-$90 range.  But this ATH-M50x had an extra bit of clarity, even beyond the best Sennheiser phones they had on display, according to my ears anyway.
 
When I got home, I immediately listened to some music I had recently mixed.  I was shocked by how strong the bass was -- to the point of booming and somewhat crowding out even mid-range material.  I went back to my near-field monitors, which are Yamaha MSP5 and the bass was quite subtle.  Granted the Yamahas have only 5" drivers and I don't use a sub, but I really didn't expect THAT much difference.
 
I also tried a set of decent ear buds, which sound a lot like the Yamaha mix to me.  I had already tested the mix through some mid-range Altec computer speakers and again, the balance was a lot like the MSP5.
 
So it really is the AT headphones that are different.
 
I wonder how common this is to find the bass much stronger in high-end headphones.  And considering that few people will be listening to the music through such headphones, do you adjust the mix based on what you hear in the headphones?  I think I could adjust the compression on the bass to keep the punch but limit the boom after the transients and that might be a little better for all listening modes.  I'll have to check the mix again, but the bass may have been running into the verb buses a little, and I could probably dial that back.
 
A few weeks back, the client for that recording project was playing the music through his car stereo and the bass was really dominant.  He liked it that way, even though I didn't think that was a very natural sound.  I had assumed he had an EQ on the car stereo with the bass really cranked.  He did have the bass cranked a bit, but I am thinking his car stereo might have had a similar response curve to these AT headphones because I had the same reaction to both cases.
 
I wonder if it is time to upgrade those MSP5s.  I suspect they roll off below 150 Hz more than I thought.

cparmerlee
I just got the ATH-M50x headphones. ...  I was shocked by how strong the bass was -- to the point of booming and somewhat crowding out even mid-range material.  I went back to my near-field monitors, which are Yamaha MSP5 and the bass was quite subtle.  Granted the Yamahas have only 5" drivers and I don't use a sub, but I really didn't expect THAT much difference.
 
I wonder how common this is to find the bass much stronger in high-end headphones.  And considering that few people will be listening to the music through such headphones, do you adjust the mix based on what you hear in the headphones?

The ATH-M50x phones are giving you a truer representation of your low end.  Think about it -- you've been using speakers with 5" woofers -- no way you're going to come close to an accurate bass on those.  
 
Considering that most 8" speakers claim to go down to 20hz -- while yours, realistically, probably go down to 55hz -- that is a huge range of frequencies you are missing out on. 
 
Yes, it may seem like cheap earbuds and computer speakers are the norm for many people, but the reality is, you don't know who will listen to your mixes and what they're listening with...  You should be mixing so that your mixes play well on all systems.  That's done by monitoring with full-frequency-response speakers, then tweaking as you also listen on smaller speakers.  You might consider getting a subwoofer to augment your speaker system, while also using the ATH-M50s as a supplement.
 
For example, I use Yamaha HS80Ms (8" woofer) as my main ref monitors, Event ALP (5" woofer) as my "B" set, Focusrite VRM box with Beyerdynamic DT770 phones, my car speakers, and an old '90s boom box to test my mixes.

JonD
The ATH-M50x phones are giving you a truer representation of your low end.  Think about it -- you've been using speakers with 5" woofers -- no way you're going to come close to an accurate bass on those.  

 
Thanks for the ideas.  I guess that is what I suspected, but it helps to hear it from somebody who has been there.  I don't think I will go with a sub because I work almost exclusively with jazz and classical stuff.   I don't do much music where a thundering bottom is expected.  And when you use a sub, there is a question of how true the LP stuff is.  I think I will look for a pair of studio monitors that are true  down to 35Hz or lower.
 
And then I'll go back and re-mix those recent projects.  I had been thinking about redoing the mastering step now that I have the new Ozone, but I really need to fix most of this in the mix, I think.

Option 1: Does anybody have an opinion about the PreSonus Eris E8 speakers?  They are selling for $375 for a pair and everything I have seen is quite positive.  The specs have them covering down to 35 Hz, which is about as low as I have seen for anything without a sub, and for under $1000/speaker.
 
Option 2: I am also looking at the PreSonus Temblor T10 sub at $400.  That one is attractive because it does have the option to use the sub's crossover to send the HPF material to the main speakers, and has a foot switch  to alternate between modes.  If I went that direction, I would keep my Yamaha 5" monitors and leave them set flat.  But I would run the signal into the sub and then to the mains with the HPF crossover enabled (crossing over at 80Hz or 100Hz.  That should give me a fairly flat response.  The foot pedal would allow me to get back to what I am hearing today, which is pretty similar to ear buds and computer speakers.
 
If you had $400 to spend, which way would you go?  (Of course, if I went with Option 1, I could probably sell the Yamaha speakers for $200 so my net cost would be about $200.)  I am leaning toward Option 2 because it gives me an easy way to hear two typical setups.

https://www.sonarworks.com/reference

S.L.I.P.
https://www.sonarworks.com/reference

That looks like a very interesting system.  I think my main problem is that my speakers simply can't produce the low frequencies, even with an artificial boost.

Those headphones do have a substantial (~6 dB) low-frequency emphasis.
 
It's generally agreed that headphones need a slight low-end boost to sound more like speakers, due to the way we perceive low frequencies when the drivers are that close to our ears. With real speakers, you get a natural bass boost as low frequencies are reinforced by the room. That doesn't happen with headphones, hence the need for a little more bottom in them.
 
However, 6 dB is bordering on too much bass emphasis, which should probably be more like 3 dB. Still, the ATH-M50s are actually highly regarded because they don't overly boost bass like so many headphones do. Compared to, say, Beatz by Dre (shudder) they're ruler-flat. But compared to more expensive cans such as a Sennheiser HD-600, they're bass-hyped.
 
So where does that leave you, besides confused? Well, knowing that the headphones are going to present a little more bass than a high-end speaker would, you can compensate for that. You never want to mix/master specifically for your headphones anyway, since few others will ever hear those mixes on the same headphones. You want to mix and master for the statistical mean, which you can now do because you know that your headphones are delivering just a bit more bass than you'd hear on high-end speakers in an acoustically-treated room.
 
In practical terms, it means that if the mix sounds overly bassy in the cans, you can take that to the bank - it really is too bassy. It means that if the mix sounds bass-weak, then you can likewise accept that as fact - it really is bass-weak. It's only in that relatively small range between obviously-too-much and obviously-too-little where the uncertainty lies.
 
Fortunately, there's an easy way around that. Tonight, strap on those headphones and fall asleep listening to your favorite commercial music, specifically records that you'd like to emulate. Do that every night for a week. I guarantee that after that, you will know exactly how much bass needs to be in those headphones to sound like a professional mix.

I have Yamaha NSM 10's.. They are well know to lack any bottom. I have gotten used to that and I know how much bass I'm going to get. I also put the LP multi band on my master buss which works like a spectral analyzer and will show me which frequencies are hot. 
 
I bought a Yamaha Sub from Goodwill for $25. They have home system subs all the time. I actually now have 3 of them, 2 are Yamaha's one is a Altec. All were under $30 and all thump out a lot of bass. 

Also, keep in mind that Closed-back headphones can sometimes exhibit a perceptively tighter, fuller bass than open-back cans. The headphones bitflipper referenced, the HD-600's, won't exhibit a build-up of bass, even naturally. What sets those more expensive ones apart is that, ideally, they aren't exactly designed to be 'pleasant' to listen to for hours. They should expose issues in a mix (if touted as 'reference') headphones. Accuracy over enjoyment.

I shot out three sets of reference style cans here and I could tell difference, even with these headsets. None of them exhibit a real noticeable bass boost. I kept the AKG k712 Pro, not really because I found them to sound the best of the three sets, but because I felt they would make my judgements of a mix much more translatable to other systems. They do pretty well, although I think vocals/elec.-guitars tend to be slightly lower in the mix in these headphones. Minor things.
 
I've used AT-M40fs cans for years, and have multiple pairs for tracking bands. They are notably stronger/fuller in the bass than the reference sets I tried. But they still sound quite good, considering. I never had any complaints--beautiful, deep low-end. I can only surmise that the M50x's are even better, and certainly for the price!!

I received the Presonus Temblor T10 yesterday.  I feed from the audio IF to the two inputs on the sub, and then output to the satellite speakers.  I had intended to set the back panel at unity gain, 80 hz crossover (as that is supposedly where the internal LPF crosses over and HPF switch set to the on position.  I thought this would give me the original satellite sound when the sub is disabled, but would take out 50-80 Hz (roughly) from the satellites when the sub is engaged.  Here is the back panel:
https://www-media-presonus.netdna-ssl.com/uploads/products/media/images/Temblor%20T10-04.png
 
The unity gain was too high for those 5" satellites.  I had to go to about 30% on the gain.  It sounds pretty good, but I really cannot hear any difference in the satellites whether I have the HPF switch on or off. 
 
I turned the gain on the sub to zero so I could hear the satellites.  They sound the same to me whether the sub is engaged or not and whether the HPF switch is on or off.  I have a Waves plug in that does sine waves at specific frequencies.  I plan to do some testing with that to see if I can satisfy myself the crossover is actually working as described.
 
All of that aside, my monitors now sound fairly similar to the headphones.  That is, either setup now highlights the problems I had in the mic below 50 Hz, so that is progress.  I had no idea a mistake in this range could mess up the mix so badly.