I call B.S. on the idea that good dynamics is a matter of distortion.
@magon I didn't say that! What I said was in 90% of audiophile conversation where the word 'dynamics' is used, you can safely replace that word with 'distortion' without changing the meaning of the conversation.
How this works is the ear uses higher ordered harmonics to tell how loud a sound actually is. This is very easy to demonstrate with simple test equipment. Ask and I'll explain how.
So if the system for whatever reason (in this case, amplifier 'X') adds higher ordered harmonics to the sound, it will be perceived as louder even though a sound level pressure meter (available as an app for your phone) will show that it is not.
If the higher ordered harmonics are a function of power, as they are in SETs, they will be showing up more on transients. Since the ear is hearing the 'loudness cues' on the transients, the result is a more 'dynamic' sound. You can read about this quite a lot with SET amplifiers, where people often comment about how dynamic they are for their apparently low power. Its simply distortion masquerading as 'dynamics'.
Good dynamics (note the emphasis; the kind that apparently both you and I like) comes from the signal and the amp will not contribute more of its own. This requires the amp to be low distortion (or at least higher ordered harmonics are masked by lower orders). A side benefit is the system will also seem more relaxed and effortless.