Bass leaves after amp warms up?

I will preface my comments by saying I have not read everything said above, so I may be covering ground that has been rehashed to death. If the problem persists after certain components have been switched out, it lies with those that have not been switched out. If it happens with different sources, it is probably not the source components. If it happens with different sources, or when a source has been switched out, or with different components in the amplification chain, that really leaves the speakers to be suspect. If one overheats speakers the sound does change dramatically. If, for example the woofer voice coil gets too hot, resistance will shoot up and the speaker will sound lighter in bass. The crossover components can also heat up and this will affect the sound dramatically as well-component values will shift and change crossover points, filter characteristics, etc. and could lead to the loss of bass.

Such obvious change in sonic characteristics, if due to overheating of the speaker, constitutes pretty rough treatment of the speakers. Perhaps something designed to produce higher volume is needed for this particular application.

Interesting, it is hard to account for all of the reported phenomena. It sound like the poster does not like the inherent sound of his system after all of the gear has properly warmed up. This would mean some kind of system tuning is in order-change in speaker and/or listener placement, room treatment, equipment change, whatever, and perhaps a change in listening habit--e.g., leaving the system on all the time so that it is warmed up from the get go.

When someone reports that bass disappears after some time, it really could be something somewhat different--it could be that the midrange or highs have gotten stronger rather than the bass getting weaker. It is not absolute level that matters so much as the balance.

Again, I will theorize that after the system has warmed up, the tonal balance has changed and the original poster may simply not like the tonal balance of his system in a warmed up state.