Bass leaves after amp warms up?

All my equipment is brand new from autuorized dealer

I too experienced bass playing disappearing act with my Audio research VT120 mono amps. It happened that when the amp is cold, the bass is not as extended but concentrated more in the mid bass so it sounded more punchy, when it is warmed up and gets lower, the mid bass hum is smoothed out and sounded like it has lose its dynamics, instead, it is more smoothed out and gets lower. My dealer told me I will hear this if the amp does not have enough current to drive the speaker. I later got a more efficient speaker and all the dynamics are back!

I'm jumping in here since I own PSB Synchrony's and I think Almarg and Mapman are correct in that your 4 ohm speaker requires an amp with higher current. At times, your speaker may even drop below 4 ohms.

mapman..."One red flag I see that might help account for what the OP hears is that his PSB speakers show 4 ohm nominal impedance and the MF amp specs indicate power into 8 ohms only."

Fishing716, if your amp is rated 200 watts/8 amps, you really would need about 350-400 wpc into 4 ohms. My amp is 300wpc and doubles to 600 watts/4 ohms; there is a ton of bass pumping out of my speakers continuously and at various volume. Try and look at amps that rate the current output, not just wpc. This spec is usually not listed, maybe ask the dealer or call the manufacturer.

Fishing716, So you have tried different speakers, different amps, different sources and different cables and are still having the same problem. The only thing left is the pre amp. Install a different pre amp in the system and see what happens. Other than than I would try a new house as maybe yours is haunted.

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.