Hi Mike, thanks for your help. I can compare the Nagra Classic to the Nagra MSA, Nagra VPA and Ayre MXR/MXR20. I had each in my system before I purchased the Classic amp. (The Ayre MXR's I had owned for ~5 years)
First up is the MXR. The Nagra Classic is more linear (even) top to bottom. More textural in the bass as well as going lower. It's also higher resolution through out the frequency range and more dynamic. The music has more energy to it. The final point is the musicality and the 3d imaging. This is the part that really won me over. The MXR is more effortless at very high volume levels, the Nagra Classic has it's limits. I have a BIG room, 25x35ft, 12 ft ceilings, and my Rockports while efficient are a tough load. The Nagra Classic is plenty fine for me. I rarely hit the limits and most of the time the Nagra never leaves class A.
The MXR-20's have more energy and resolution than the MXR's. Better Bass too. On the surface, they are simply better amps than the prior MXR's, however I felt the MXR20's missed that musicality that the Nagra classic possesses. The Nagra is more interesting to listen too. (at 1/2 the price I might add)
The Classic vs the MSA, while the MSA has that Nagra musicality and is incredibly wonderful to listen to (like the MXR's), the Classic simply presents more information (resolution), presents it better in space, unravels the music better, and has more power. If you love your MSA's keep em. Great amps. The Classic is better however.
The Nagra VPA's were a harder decision. Incredible presentation of space, unravels the music nicely, never runs out of gas, but they in comparison to the Nagra Classic don't quite have the same resolution and bass control. The VPA's do present more wetness to the sound. More weight on the notes if you will. Those 15inch drivers on my Rockports just "locked in" with the Classic in way that I couldn't let go. Again the Classic is ~1/2 the price of the VPA's.
Now that I've had the Classic amp for about 3 months, it just keeps drawing you into the music. I listen to jazz, light classical, folk. If I was into full out orchestra's I may want a 2nd Classic for the extra power or some other amp. I'm quite smitten by the Classic amp. I think it's a steal for the money considering I choose it vs 2 other amps at ~30k.