As I indicated in my previous reply, neither my personal experience nor my knowledge of the principles of audio technology incline me to place a higher priority on amplifying equipment than on the speakers. One of the great things about the current audio scene is the availability of receivers(such as your HTR-5990)at modest cost which amplify signals with audible transparency within their designed power limits. So, I can't offer any help as to more exotic amplifying units.
My own listening is probably close to 99% music, but I never listen to 2-channel source material only on front speakers. If I followed your comments, you apparently listen to music using only the M60s. If so, my practice and suggestion to you is to use surrounds such as the Axiom QSs with a mode such as DPLII. This extracts the surround ambience that was picked up by the mikes and mixed into the front channels(there was no place else to put it)and sends it to the surrounds where it belongs, making for a more realistic listening experience.
Again, my view is that this is the way to go, rather than spending money on amplifying equipment.
