Loss of bass & full spectrum sound

A few suggestions:

1.
Use 10 gauge or 8 gauge speaker wire. Get the Belden 5T00UP (10awg) or the Canare Star Quad 4S11 (8awg). The thicker cable will offer less resistance than similarly designed smaller gauge cable. In my experience, I get deeper bass with the larger gauge cables. If you're bi-wiring, get a pair of speaker cables per speaker, even if you get the Canare 4S11.

2.
If you have a source with Balanced output, use it instead of RCA.

3.
Get to audition an isolation transformer type of power conditioner, such as Torus Power or Bryston BIT units.

I'm also in Manhattan and I have experienced something similar some years ago with another system. I did some experimentation a few years back looking into the effect of speaker wire gauge and bass response. Using cheap speaker wire, I found out that as the wire got bigger (smaller gauge), I got deeper bass and a more balanced sound.

Also, in my system, I get better bass response using copper wire instead of silver. Silver provides better definition, but I get a more balanced sound from top to bottom using copper-based cables.

I hope this helps.

Not likely. But I agree with everything you've said. IME, you really do need 8awg, all the way to the power supply itself. Change it out in your amp and you'll be blown away.

Is it possible that the speaker drivers have a defect which shows up after they have been driven for a while?

You might also try a different outlet using a 12g extension cord. You could also go as far as trying an outlet that is on a different breaker.

That's interesting
I had PSB Synchrony II & figure Synchrony would give more bass (which it does when working) but never tried different brand speakers.
I like PSB Synchrony sound and no one else has problems with them.So would probably discount a driver problem with the PSB