I've had the this rig for about 2-3 months using the stock tubes of the MG along with a 30 minute warm-up period. I've averaged about 3-4 hours of listening a night during that time, so I've become accustomed to the sound of my rig. It offers up a warm, neutral sound, very detailed, crisp, non fatiguing highs, tight, but not overpowering bass especially with jazz and classical. Within the last 2 weeks I've changed the tubes:
MG Head - Mullard CV4004 and a pair of Yugo (Ei) El484s)
Senn 580 (w/ Clou Red Jaspis cable)
Beyer DT 250-80
ICs are Nordost Solarwind
This particular tube set-up with the 580 is better suited for pop/dance/trance/ambient, etc. It has more bass extension, but yet retains great detail, still without being fatiguing. I can't wait to get my 325s and see how the Grado sound affects the rig.
I replaced my 10 year old Sony with the NAD. I did a lot of research on the web for CDPs in the $400-$600 range, I learned a lot, but nothing replaces auditioning. I spent 2 hours at Holm Audio with a similar system NAD c521, MG w/ stock tubes, Senn 600, and Nordost Red Dawn ICs (~$350/ meter!). That system had more bass (Massive Attack's Angel sounds great), but the highs were a bit too much, bordering on shrill, especially the horns on Copland's Fanfare for the Common Man. I slipped on my 580s and everything was right with the world. I did not need to hear anything else, though the ICs were a bit too much and bit too revealing. Albert recommended the Blue Heaven or the Solarwind for a bit less detail and less money. The improvement over the Sony was the proverbial veil being lifted from the music.
I must admit that I've been bitten by the tube rolling bug, but as I said I became accustomed to my rig (daily listening for about 2-3 months with various music) with stock tubes before I made any judgements as to what needed refinement. If you don't have the cash or patience, the stock tubes do a wonderful job. I thoroughly enjoy the music that flows from my rig.
Buy the MG Head from Headroom. Ask Todd some questions about the amp. He is a great guy working for a great company!
There are other positive reactions to the MG/Senn 580/600 combo. This, of course, is mine IMHO.
Regards - reynman