Bass leaves after amp warms up?

I only have these speakers

The bass is firm & lasts longer with just one speaker connected.Eventually leaves but right now it is working for about 30 minutes
Listening to Chicago V on Rega P5 TT w/dynavector.
I would keep this sound w/one speaker & loss of half information.
But soon it will leave.
I am monitering the situation

Please let me know if anyone notices contradictions in my observations.
As I've tried everything
Could it be reasonable that loss of deep bass is normal?
Perhaps I need to augment my system with a subwoofer.
I feel bass is adequate when I have it

FIsh,

It is what it is but for future reference, its not a bad idea to always keep a spare component around to switch in as needed to help diagnose a problem or even to pinch hit when a starter piece goes down.

For testing purposes, an inexpensive receiver or integrated with preamp outputs and amp inputs can be used to replace another integrated, power amp, or pre-amp when needed. Cheap spare CD players are readily available these days, just check out the local Goodwill store. Cheap spare speakers can be had easily off Ebay, as can most home audio components.

This way you can do "regression tests" to help isolate a problem to a specific component when needed.

Subwoofer would be a band aid.

BEtter to get to the root of the problem and fix it. The PSBs should leave nothing to want if set up well.

IF either speaker alone sounds better, that would seem to indicate the problem is not in either speaker, like detached/loose voice coil or something like that.