Bass leaves after amp warms up?

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.

Short of moving some of your equipment to another location, you might try monitoring the apartment voltage and seeing if there are any dips happening when sound goes bad. A hand held RFI detector is also a good investment.

Lets get back to the electrician visit....

When the electrician visited, did he monitor the power before, during and after you had these issues? If it included during your problems (what would be the point otherwise?), did he not hear the issues too?

But this time I had the power conditioner unpluged from the wall.

On the 3rd page of your thread, I posted:

"-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."

Intentionally or unintentionally I think you are just wasting everyone's time.