Bass leaves after amp warms up?

Atmas,

WHy rule out the amps when there is nothing indicating any of them are/were up to the task of driving the load of the PSBs without running out of gas.

RF from digital, malfunctioning or otherwise, might have an effect but my experience with that is effects are noticeable from the start and not cumulative over time. Not to say any listener might notice ANY negative effect more so over time in general.

Regardless, testing with a different source to see if same results if possible would be worthwhile.

Can't you get your dealer who sold you the amp and/or speakers to help you sort through things?

My dealer thinks I'm a kook or have hearing/mental issues..
The support @ PSB are even worse & dismissive.
I would say for the record they are rude with no patience,interest or respect
That was my experience with PSB phone support.
I would never recommened this brand to anyone even though I like their speakers..

Here's an additional thought for the OP to consider:

In my experience as an electrical engineer, when a baffling problem occurs that defies diagnosis by the usual methods, including substitution of relevant hardware, it often turns out that TWO problems are present at the same time, both of which happen to produce similar symptoms.

Not sure what this might mean in this particular situation, but it's something to keep in mind.

Regards,
-- Al

"What happens if you listen to an LP, and not only have the digital off but entirely disconnected, even unplugged from the wall. This is sounding more like an RF interference problem, and malfunctioning digital can do that"

I have not tried to unplug digital
Video sources loose their oomph too:

The soundrack on a James Bond DVD will go from viceral groans in the backgound that heighten tension to just background sounds.
Voice loose depth & personality
These are world class $5500 speakers highly rated by everyone.
They have (3) woofers to support solid bass.

Tell him the gang on A'gon does not think you are a kook and that we will boycott his store (wherever it is) unless good customer service is demonstrated! :^)

Good customer service including ability to audition gear when needed is pretty much the only reason to buy audio gear from a brick and mortar real store any more.

Does the dealer still have the PSBs set up?

If so, maybe listen to them there (or anywhere else/another dealer if possible) and see if you hear anything similar to what you hear at home. Take note of the amp being used and investigate its specs versus yours in terms of ability to drive loads of 4 ohm or less. See if there is any correlation between what you hear there and what you hear at home and use that as a reference point for deciding what to do from there.