As I understand it, an Ultra will put out a bit more energy than a 600 at the lowest notes and highest volumes. Where the 600 pulls ahead is "real world loud playing" -- you can run the system at a level where the Ultra would be overdriven by the lowest frequencies (as would a 600 without DSP) and the 600 sounds fine because it automatically cuts back the volume when necessary -- but 99% of the time it isn't necessary. As a result, movie and music tests seem to give the 500 and 600 a slight advantage over an Ultra despite the Ultra's better driver.

M60ti, VP180, QS8, M2ti, EP500, PC-Plus 20-39
M5HP, M40ti, Sierra-1
LFR1100 active, ADA1500-4 and -8