Bass leaves after amp warms up?

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.

Have amps always had this issue? DId any work good initially at least for some time before the pattern started? ANy nasty sound accidents prior, like loud feedback due to improper connection, or anything else similarly abnormal?

Fishing716, it could definitely be the amp or electricity as others have said, but with all respect the loss of bass weight and "flatness" you describe does correspond very well to the changes in hearing that come with the acoustic reflex. I'm not saying this is absolutely the issue--just that it's worth considering, especially as you've had the same experience with multiple different speakers and amps.

Note that in Tyll's InnerFidelity article he hypothesizes that the acoustic reflex is one reason systems tend to sound edgy and bright at audio shows--a pretty reasonable notion when you consider that it creates a subjective 20 dB bass roll-off (check out the graph). He notes that his reflex kicks in at 55-60 dB with pink noise. This is probably a bit different for all of us, but for me it's about the same. It's one reason I'm also (like you) a low-volume listener most of the time--though for me that often goes up to 75 dB or so.

As an experiment, you could try starting with the volume turned all the way down and turn it up till you just begin to feel the bass weight (this could be at an even lower volume than you usually listen at), then listen for a while and see if the same thing still happens or if it happens as quickly. I mean no offense, it's just that we usually (myself included) look first at the gear itself for problems--a lot like when someone keeps buying new speakers w/o paying attention to placement with the speakers they have (not your situation, just an analogy).

Fishing, is it related to the time of day? Early mornings and late nights it sounds good? Middle of the day, it sounds bad? I've noticed my system sounds better late at night, around 9pm and later.

Also, are you able to demo another amp from a local hifi shop?

If none of that works, maybe you should sell me the psb's at a highly discounted rate, since they're obviously defective ;-)