Bass leaves after amp warms up?

Although you don't listen loud, it could still be the acoustic reflex changing how your ears perceive sound--the net effect when our ears involuntarily "tighten up" is a loss of sensitivity to low frequencies and a perceived emphasis of treble frequencies. Your _body_ cuts out the bass. Check out the links below.

If your system tips at all toward being "bright" or "forward" triggering this reflex could be possible even at low volumes. Though my system is on the warm side, the acoustic reflex kicks in sometimes when I listen as low as 60 dB if a CD is poorly mastered or if I'm just plain tired or stressed. (Atmasphere's discussion of odd-ordered harmonics and intermodulation distortion in another thread might be relevant here, too: I have an ss amp.) My car has 70 dB interior noise on the highway--again, not that loud--and my acoustic reflex kicks in there, too. It's one piece of the listening-fatigue puzzle.

https://en.wikipedia.org/wiki/Acoustic_reflex

http://www.innerfidelity.com/content/loud-music-sucks

In short, there's always a chance it isn't purely the equipment, but your ears. On the other hand, changing equipment (say to a less "forward" amp or speaker) can change how your ears respond to the sound. Grado headphones (forward) wear me out in minutes at 60 dB, but I can enjoy my Sennheisers (warmer) at 70 dB for much longer periods.

Atmasphere's idea to walk away for 1/2 an hour with the system on is a good one but also might reset your ears as much as anything.

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).

Agree with Newbee on using an SPL meter and LF test tones to gather data.

Beating my own dead horse here, but I suggested it 200 or so postings ago and it could still be a piece of the puzzle. It goes with what Jjrenman said above about listening fatigue. This problem (especially if the OP's system sounds at all fatiguing) could be entirely explained by the acoustic reflex--the natural reflex our bodies have to surprising, loud, or unpleasant sounds that mechanically diminishes the conduction of sound to our inner ear. I'll quote from Tyll's "Loud Music Sucks" article on Innerfidelity:

"Your listening system has built in protection from loud noise called the acoustic reflex (also called the stapedius reflex). When you are exposed to loud sounds, small muscles (stapedius and tensor tympani) in your middle ear tense to increase the mechanical impedance of the system of small bones (oscicles) that move vibrations from your eardrum to your inner ear. The net result is a reduction in amplitude of sound being transmitted into the inner ear of up to 20dB at low frequencies, with less attenuation as frequency rises up to about 2kHz where no attenuation is seen."

Please note that he also hypothesizes that the acoustic reflex is why so many people describe show systems as bright--their bodies are mechanically filtering the bass out of the (too loud) sound.

http://www.innerfidelity.com/content/loud-music-sucks

20 dB is a HUGE difference in the bass and having this happen would absolutely result in a perceived loss of bass impact but would NOT mean anything is wrong with your hearing. On the contrary, this would mean your body is responding as it should to loud sounds. Apart from measuring LF test tones, this is also where an SPL meter comes in handy. You say you listen at low volumes, but low for me is 50-60 dB with 80-85 dB my max. For others 80 dB might feel like a starting point. As I said before, I can feel and hear my own acoustic reflex kick in at volumes as low as 60 dB if I find the sound of the system (or natural sounds around me) unpleasant. If your SPL meter shows an objective reduction in bass output, I will gladly cheer on an appropriate resolution to your problem. However, if the SPL meter shows the bass is still there but you're not hearing it, the acoustic reflex is on the table.

You have great equipment which I'm sure sounds very good, but certain components have been accused of brightness and in combination (at whatever volume) could be making you subconsciously cringe as this reflex kicks in. Again, Fishing716, I'm not saying it's all in your head or that there's anything wrong with your hearing--just that there is a possible explanation outside of swapping gear. And if it IS the acoustic reflex, you might start looking for gear that sounds less fatiguing.

One more idea: your hearing might be exceptionally good. The PSB Synchrony models have an unusually low spike at the tweeter's resonant frequency, around 16-18 kHz (or maybe this is simply a spike in the treble apart from the resonant frequency). It's possible that (like me) you're still young enough that your hearing extends up this high and you're bothered by the spike, finding the sound fatiguing. From JA's review of the Synchrony One:

"The top octaves sounded smooth to me on this Telarc SACD—the delicately brushed triangle at the end of the final variation before the fugue was beautifully resolved, without sounding spotlit—but Erick Lichte was less tolerant than I of the PSB's performance in this region. However, in the "Measurements" sidebar accompanying this review, I wonder if he was reacting instead to the small response peak between 16 and 18kHz, which, unlike me, he could hear."

http://www.stereophile.com/content/psb-synchrony-one-loudspeaker-measurements

This could create listening fatigue for you and possibly (my dead horse takes a further beating) trigger the acoustic reflex--further creating a perceived tipped-up tonal balance.