Bass leaves after amp warms up?

I've heard the PSBs sound very good off 100w/ch Rowland Class D switching amp. That would be a good place to start as a reference. I truly believe a good modern Class D amp with good power supply that can mostly double down to 4 ohms can solve OPs problem in a relatively inexpensive and very compact package.

Rowland 100 w amp for reference.

I would ask suggestions for more efficient integrated amp to drive PSB speakers..

There are many well received integrateds out there these days that use CLass D amplifiers, even inexpensive Japanese ones from companies like Onkyo. Quality of power supply, which matters might vary widely with an integrated more so than separates. If the specs indicate mostly doubling down to 4 ohm, it is probably worth consideration. There are still limitations with amp and pre-amp in close proximity in same box with Class D, but technical aspects relating to noise and shielding are different. I have not used Class D integrateds so cannot vouch, but many have received good reviews and acceptance. To be safe, I'd say separates are better (and with Class D amps smaller as well). But why not try a well reviewed integrated for reasonable cost first and see? If it does not work out then move on eot separates. I think there is a decent chance the right integrated could work for OP.

The voltage is normal
By the end of the day there is nothing left in the bass dept.
I turn off the amp for the evening and return in AM
Plenty of deep bass & sounds excellent
I listened to Abbey Road this morning but by the end of the first side
McCartney's bass lost his groove.
I didn't bother listen to the second side.
Sometimes I switch speaker cables from L/R to R/L
And lower bass returns for an hour or less.
It's quite a dilemma