I was sitting here listening to the P1u when you posted the caps/toenails comment, and noticed this post about quietness. The P1 is a silent, silent amp, and I had the same reaction as you. I have both of them hooked up right now to the same balanced source and nothing in the RCAs. The P1u has the plastic caps, but otherwise they are set up the same. I paused the music and listened on both amps with the HD800s.
At 10:00 on the volume, neither was making any noise--no hiss, or what I call "presence" when you can just hear enough to know it is on. I then cranked the volume all the way on the P1u and there is only the slightest hiss/presence audible. It is there, but it is very, very faint. Also, the "sound" does not ever appear until 4:00, which is almost fully open (5:00 is the end of the dial). I then did the same on the P1 and it was FOUR TIMES LOUDER! Well, I'm kidding, but it is noticeably louder even though it is still very quiet compared to many amps. It also kicked in around 3:30/4:00 on the dial. Interesting. I also switched the input selector to coax and there was NO noise at any volume level on either amp. Pretty impressive.
I don't know where to get the plastic caps for RCA and/or XLR. Elephas gave me some metal RCA plugs, and I know Cardas makes that type.