[Music Alchemist]

  And just for reference, http://www.innerfidelity.com/images/SennheiserHD650.pdf
 
The 650 is practically ruler flat from 60hz to 1200hz. The M50x is visibly 8-10db from 30-100hz over its midrange response.
 
Everyone's ears are different, but the 650 is an objectively very flat headphone and the m50x is objectively, extremely bass heavy.

 
The HD 650 sounds somewhat warm, actually. You can't just glance at the graph to assess things; you need to superimpose compensation curves over it. I may share one for that headphone soon.
 
If you want linearity in the bass and lower mids, some of the high-end planar magnetics measure nearly perfectly in that respect, but have issues further up.
 
Here is the most neutral (across the spectrum) headphone I have heard/seen:
 
http://cdn.head-fi.org/6/6d/6de6f4f3_STAX_SR-207_EP-507_SB2217.png
http://www.innerfidelity.com/images/StaxSR207EP507LeatherPadsSerNumSB22217.pdf
 
I do agree that the M50x is far bassier than the HD 650.

 

[Sajakhaki]

I would say the 400I or even the 400S with the decent amp is my favorite all arounder

 

[DivergeUnify]

Used to be the Hifiman 560, a very good jack of all trades going for $500-600 used

 

[cocolinho]

  HD650 with a decent dac/amp . Veil is a legend

I would also add Hifiman HE500

 

[malazz123]

i got 
 
- sony mdr1a
- ath ws1100
 
the latest bose soundlink 

 

[Kodhifi]

There is some misunderstanding here. I'm not talking about frequency graphs or measured results. I'm talking about putting them on your head and pressing play. The headphones have a similar sound to them. If you don't own both headphones and are arguing based on a frequency plot then you have no first hand experience with them. I didn't say they are neutral, I didn't say they are ruler flat. I said that they are voiced similarly and that is the impression I got listening to them with a variety of music.
 
Of course closed backed headphones have a different sound stage, and usually have more bass, but that's not what I"m talking about. In fact if you haven't heard the headphones, it will be impossible for you to understand at all.
 
the M50x takes EQ very well and it's sound can be tailored to a persons preference. In fact there is already a profile for this headphone in sonarworks and selecting a different output curve transforms the sound of the M50's into AKG, or Sennheiser. Again I'm not talking about measurements or theory. I have these headphones, I have used them, I have eq'd them, I have used sonarworks with them and I am talking from first hand experience.

 

[Music Alchemist]

  There is some misunderstanding here. I'm not talking about frequency graphs or measured results. I'm talking about putting them on your head and pressing play. The headphones have a similar sound to them. If you don't own both headphones and are arguing based on a frequency plot then you have no first hand experience with them. I didn't say they are neutral, I didn't say they are ruler flat. I said that they are voiced similarly and that is the impression I got listening to them with a variety of music.
 
Of course closed backed headphones have a different sound stage, and usually have more bass, but that's not what I"m talking about. In fact if you haven't heard the headphones, it will be impossible for you to understand at all.

 
I have owned them, and I even had two pairs of the M50x. (Perhaps consider reading someone's profile before making assumptions.) They sound nothing alike. The overwhelming bass of the M50x drowns out the mids while the HD 650 is fairly neutral with some added warmth and slight darkness. If the bass of the M50x was toned down, it would be fine, since the rest of the spectrum has no major issues. The sound quality of the HD 650 is much better, regardless of FR. I think the sound quality of my $15 Koss KTXPRO1 is also better than the M50x overall.
 
Here's that HD 650 graph I promised. As you can see, my subjective impressions align with the measurements.
 
http://cdn.head-fi.org/5/5a/5ac3a0cc_Sennheiser_HD_650.png
 
To be fair, I guess I can understand why you feel they're tuned similarly.

 

[TWerk]

DT1770 - Has excellent balance, but still fun, detailed, resolving. Comfortable and easy to drive. Great bass. Perfect all rounder.

 

[DivergeUnify]

lol, the M50x and the HD650 sound nothing alike. HD650 have godlike mids and a very laid back but revealing sound
 
The M50Xs are, in comparison, a sucked out mess.  They really aren't in the same category of sound signature, let alone quality

 

[stuck limo]

The ATH-m40x I own would fit the answer. They handle everything very well and are not offensive in almost any way.