I had used the ATH-M50 exclusively for several years until recently I got the bug to upgrade. I first tried the NAD VISO HP50, which initially did not impress me as significantly better than the M50. After careful and prolonged comparison I found that the HP50 did sound better, but eventually I returned them and bought the HD600 instead.
In contrast to the HP50, the difference between the HD600 and the M50 was immediately obvious. The first CD I happened to listen to with the HD600 was Catch a Fire (Barry Diament mastering) and it was as if I had never heard bass before. I cannot understand why people say the HD600 lacks bass, because the bass is usually what I'm most aware of in most of the recordings I've listened to so far.
Right now I run the HD600 from the headphone jack of my 15-year old Onkyo receiver TX-8211. It sounds pretty good to me, but I have no experience with other equipment, so I wouldn't know if it were lacking. Over and over I see people say "The HD600 need a headamp" but I get the impression that these people assume your source is a laptop, DAP, or cellphone. I run my CD player and DAC through my receiver, so I'm wondering how much of an improvement, if any, a dedicated headamp like the Magni or O2 would actually be over the receiver's headphone out. Does anyone have experience/advice on this subject?