Bass leaves after amp warms up?

Is this plausible?
1. You bought your PSBs used,
2. Whomever you purchased from blew out the woofers and had them reconed.
3. The new surround is nice and springy when you first start listening but "poops out" when it gets warm,
4. Most amps put out more power after they warm up which contributes to the problem.
Problem solved!

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.