Here we go. Congrats on what is a great amp, brutally revealing but very rewarding when coupled with a proper source.
Careful, some those combinations are in no way guaranteed to play nice with your PPA depending on how it's configured. Specifically if the gain setting could mean very bad things for the 637s, not too sure about the others.
Unless you have reason to believe that something is setup incorrectly I'd leave well enough alone. Take it from someone who has rebuilt PPAs after people have blown them up trying to adjust the buffer bias, you DON'T want to find yourself in that situation. It took 3 rebuilds to get one of them working again. And it's really easy to have a probe slip and short the buffer while trying to adjust the bias and then all your amp will do is smoke components (how I got my custom title at head-fi) which is not nearly as much fun as listening to music.
I'd highly suggest leaving it that way (see below).
http://www.tangentsoft.net/audio/ppa/amp2/ - read all of that, then re-read it, then go read this - http://www.head-fi.org/forums/f6/ppa-v2-help-smoking-output-resistor-long-124880/.