Bass leaves after amp warms up?

Agree with Newbee on using an SPL meter and LF test tones to gather data.

Things to try:

-measurements with sound pressure level meter and test tones
-confirm the year and results of your last hearing test and those of anyone else who hears the same thing (just because your S.O. hears the same doesn't mean anything. They might be an ex-rock musician with years of 110 dB experience, for all we know) I believe you, but with your musician background this should be confirmed before the thread reaches a few hundred posts.
-power down the whole house (including tenant)using breakers except for the outlet for audio. If there are other items on other outlets running off the same circuit unplug them. Plug just your amp directly into one wall outlet and just your CD player into the same wall outlet. Retest. If still bad, try moving your amp, speakers and CD player to another circuit and repeat.

The OP isn't willing to try much.

SPL meter? Nope. He's a professional musician, won't try the SPL meter. Taking the cover off the amp? Nope, no tool. Unwilling to go to the store to buy a set of allen keys. Besides, he isn't good at taking the cover off, he has the dexterity to be a musician but he can't turn a screw.

(Hardware stores will have both US and metric sizes by the way -- really easy investment.)

Likes swapping speaker cables though.

Refrain from responding Wilsynet
As your attitude speaks volumes

There's 148 posts to this thread and I haven't read them all but I know that's it's been brought up. I just don't know what your response was.

Why don't you at least try a power regenerator like a PS Audio unit to rule out any chance that it's a power problem from high usage? That way you know that your power to all of your components is consistent.

Chuck