Well - a few initial thoughts.
For music I put the medusas in stereo mode and plugged them into the same Sony stereo amp as the HD600s/ The HD600s are a real treat for music - much better than the medusas. I can best describe it as being like putting on my glasses. But here's the thing. I'm not that short sighted. I pretty much sit on the boarderline of legality for driving. On a sunny day I can read a numberplate at the required distance - on a dull day I have to squint. The difference isn't like night and day but it's a real improvement.
The HD600s are like being able to see that extra detail - like seeing the mortar between the bricks on a distant building. Musically it's the difference between hearing the note and feeling the character of the instrument. You can hear things like body resonance on a guitar that just aren't apparrent the Medusas. Echoes sound distant from the head. Delicate sounds can be heard in more detail. The HD600s have a much better dynamic range. Quiet sounds are just as good as louder ones. On the medusas you can usually hear the detail but you have to turn up the volume and everything sounds flat and dull by comparison.
Have to say I'm hugely impressed with the Xonar / HD600 combination for non-binaurally recorded music. With dolby headphone it sounds like I'm sitting in front of a great stereo system rather thanlistening to headphones. Very natural sounding. I even took the headphones off to make sure the sound wasn't coming from my stereo speakers.
Onto gaming. The Medusas were moved over to their own amp, while the HD600s were still plugged into the Sony amp. Now here's the catch for me. I still don't find CMSS-3D or Dolby Headphone as convincing as the Medusa's directional sound - even on the HD600s. It's really the algorithm that makes the difference - not the headphones. I find that the HD600s give a reasonable sense of space (more so than the medusas) but not so much of direction - at least when it comes to front and behind. Sure - the HD600s sound better quality wise - no question. Tried HL2 and Crysis with both the X-Fi and the Xonar - with both sets of headphones. The Xonar seems to have problems with 5.1 in HL2 but in Crysis it's stunning. The 5.1 positioning on the Medusas is notablly better on the Xonar compared to the X-Fi . That sense of it being too wide and not deep enough that's with the X-Fi is gone.
A word on positional sound. I suppose the Medusas give a better (Edit: no not better clearer) impression of whether things are in front of or behind me. The HD600s with Dolby Headphone give a greater sense of space. If anything - the Medusas exaggerate the directional component to the sound. It's less natural sounding but more obvious (Edit: like the difference between 5.1 speakers and real life - I could see the former giving a competitive edge though).
I should mention what a pain it is to get the Medusas sounding their best. It's pretty obvious why so many people have given up on them - without going to a lot of trouble they are pretty awful. I think I've done enough to get them to competent but unspectacular level. I have the medusa amp standing on its side next to my hi-fi amp (to give a little EM shielding), a ferrite ring on the cable and the in line controls set to as close to minimum as possible. That's just to get rid of interference from my Wi-fi. The volume is controlled on the medusa amp. The DVD / CD switch is set to DVD mode. Bass crossver frequency on the soundcard is set to 100Hz. Even with all this you can hear distortion on better quality source material.
For music I've found it's better to ditch the Medusa's amp entirely. Switch the DVD / CD switch to CD mode (which spreads sound accross all drivers and redirects the bass to the sub channel) and plug in to a better amp. Most of the distortion problems disappear, which proves what a dreadful piece of kit the Medusa amp is.
So what will I be doing in future? It may surprise some of you but I'll be keeping the Medusas for gaming (Edit: longer term I could see myself getting used to Dolby Headphone though) and I'll even listen to music on them for the time being . They will (Edit: probably) remain my second choice behind surround speakers for gaming. I will definitely look into getting a better set of stereo cans in future but don't fancy spending £200 odd on them. I may look into one of the sub £100 Goldring, Grado or Sennheiser sets.
Edit: This has reminded me how subjective audio can be. It amazes me that so many people swear blind that 5.1 headphones do not work. I think the truth is that they may or may not work, depending on the individual. It's pretty certain that the quality just isn't good enough though.
