Bass leaves after amp warms up?

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.

Larryi, You just duplicated my post from the beginning of this thread and after 150 posts I still believe this is the OP's problem. Tonal balance is a delicate thing and it doesn't take much to throw the sound off.

The lower octive has depth & moves air when it is playing optimaly..
After a time that quality receeds in to the background and the fundamental tones are no longer satisfying

I had Electician over today & found nothing wrong with phase,ground,connections or voltage

I think you should have Johnny at the Audio Connection loan you a pair of Vandersteens to see if a warmer speaker cures your problem.

I may have missed it among the 140+ responses, but did anyone suggest you run an extension cord from another part of the home to the amp? Perhaps the circuit breaker or the wiring/panel/outlet has a resistance and after an hour of high current from the power amp the thermal resistance is increasing, therefore limiting the bass notes. Notice that the phenomenon took longer with only one channel driven ( as the single channel driving a speaker will draw less current from the wall, but problem still occurred later, nonetheless). Adding a powered sub to the circuit will exacerbate it as another amp is being shared with the AC, no?

3 amps with exact same issue rules out the amps for sure, plus as I used to repair Musical Fidelity, I know the M6i runs very cool (if properly biased), and is Bipolar transistor powered. Removing the cover would be not help if the amps cover is cool to the touch. Plus, I know the M6i is fine with 4 ohms; all day long.

Hope this helps.

Steve