I had an urge to compare all my headphones tonight for some reason. (This is not a scientific test)
Playing through the following:-
Marantz CD6000 Ki Sig -> Musical Fidelity X-Can v2 powered by X-PSU.
Anyone who is unfamiliar with the X-can, its a tube amp design.
Headphones
Sennheiser HD600
Sennheiser Momentum
Sennheiser HD25-1-ii
Sony MDR-7506
Sennheiser HD495
Basically, after an hour of swapping between them, I put the HD600 and Sony 7506 neck and neck. If push come to shove, the HD600 wins but I did have to turn the volume up 10% when I plug it in as it is harder to drive.
The Momentum is actually most disappointing. In isolation it sounds great, very detailed, nice bass but then you put on either the HD600 and the Sony you realise the Momentum is actually not as open in sounding, it's like the higher end frequencies are muffled a little, its a little but noticeable.
The HD25-ii are good but the least comfortable of the lot. The 495 is the worst of the lot, by quite a margin. It is just not as detailed, full stop, in every regard.
Basically after a while I realise the Sony and the HD600 are the best of the bunch. The kicker is the Sony set me back about £70 and the Momentum was £150 ! The Sony is also more comfortable, just as easy to drive with a more open sound. I am surprised how open the Sony sound considering it is a closed design. The HD600 is truly open sounding, for headphones wise the sound sounds like they are coming from further away than it is (but nowhere near speakers like obviously).
I was a fan of the Sony 7506 before but this is the first time I ever sat down and compared them all back to back. I really don't know how they sell these so cheap considering the HD600 were £300, the Momentums are £200+.
Headphones not tested - HD580 (broken cable and worn ear pads), Goldring NS1000 (AWOL somewhere in the house).