Thanks for the info on that thread. I had not seen that, although I did a lot of research. I was more concerned about the A23, as there had been the occasional mention of a hum or hiss. Almost always, as the threads progressed, the sound turned out to be related to the circuitry external to the amp and not the amp itself.
I'm glad to report that the A23 I received is absolutely silent, not even the hint of whisper with my ear pressed against the tweeter and with the preamp volume turned up very high.
As far as the P5 preamp goes, I see that the thread you referenced ended 18 months ago, so I hope that if a systemic problem was identified, it has been rectified by Parasound. So far, the P5 I have has been exemplary.