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My reply on your thread answers most questions.
I believe 1980s Perreaux mosfets were Hitachis.
I use two Dreadnoughts. Prisma 750s are not common ground. My SRSs required a Dreadnought. The Dreadnought in my PMF 3150 rig improves the sound.
Perreaux amps play well at low volumes. Vintage Perreaux preamps do not provide many low-volume settings from CD players -- 3-5 clicks raise volume too high. I bought a used SM6P to control my PMF 3150.
All the PMF series amps, as far as I know, are common ground.
In my post on your thread, I provide information about pricing. I have not seen a PMF 5150b on eBay or Audiogon. A PMF 5150 recently sold for over $1,500 on eBay. The PMF 5150 would likely command a higher price than a 5150b because the "b" series is, I believe, the professional model, which includes a fan, and likely has somewhat less precise sound reproduction.
I love the Perreaux/Polk SDA combination and would recommend it to anybody.