Bass leaves after amp warms up?

Your conclusion is very on point..I came to the same one but was "convinced" by Dealer that wasn't likely.
You would suggest I get 8ohm speakers?
And no one else on this planet had similar problems?
PSB Said "we sold many thousands of this model" not even one similar complaint.

Ralph, I have 4 Acoustat TNT amps just rebuilt by Roy Esposito. They are TO2 mosfet amps. I assure you they get better as they warm up. So much so that they are 'eerily' life like when in mono mode. I don't know what Roy did, but I have not experienced this kind of sound in my life. However, may not have anything to do with your assertion.

Thanks Atmasphere
You're really smart,I will follow your suggestions & report back

In addition to the experiments that Ralph (Atmasphere) suggested, if readily practicable it may be worthwhile to try running the amp with its top cover removed, which would lower its internal temperatures.

You would suggest I get 8ohm speakers?

If you are happy with the sound of the speakers during the first hour, and if the conclusion is reached that either the speakers or the amplifier has to be changed, it would probably make the most sense to either change the amplifier, or to purchase a separate power amplifier and drive it from the M6i's pre-outs.

Regards,
-- Al

I go back to my original assertion. As Almarg says the results suggest that these amps could not hack it with these speakers (not a good match into a 4 ohm load). I would look at trying an amp with the PSB'a that doubles its power into 4 ohms. (high current)