Bass leaves after amp warms up?

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.

Acoustic reflex seems like a natural defense system against what I have read where 80-85 db is the general limit for safe listening without causing damage to hearing.

It is very possible that when sound seems to change systematically over time, at least part of it is due to our hearing senses adapting or changing as needed.

We tend to think of many things like this as a constant, but no doubt people react differently to things at different times. WHy would hearing be any different?

Thanks to the PSB owner for chiming in. I do not doubt both amps used are underpowered to drive the nominal 4 ohm load. Whether that accounts for all that is being heard is a different story.

One might think any 200 w/ch amp is up to the task of driving most any speaker optimally (not just well). One would be wrong. I've had 360 w/ch and 180 w/ch amps that cannot.

The current delivery and ability to double down at 4 ohms matters. If the OPs MF amp could do that, it would likely warrant being in the published specs. That and being a conventional Class A/B type integrated, which most always has physical constraints that limit what can be done with power supply in close proximity to pre-amp circuitry, I am very confident that it does not.

I disagree with hearing perception changes
Simply put the bass when working has punch,heft,depth & moves air while creating excitment & emotion.
After about an hour the bass notes are still there but the kick of the bass drum no longer has the correct punch,the bass does not move air,the bass notes are no longer rounded but flat,etc.