Do you have the preamp's that are supposed to go with the PR-2100's. Those are custom modified. The original PR-2100 did not have knobs on the front, the PR-2100 type II had the frequency adjustment knobs on the front. They also did not have RCA inputs, it was a single 5 pin DIN. The PR-2100 was basically two PR-250 housed together in one chassis designed to be bi-amped. Way ahead of their time, there were only maybe two cassette decks that were preamps only when the 2100 came out. The Alpine 7307 and the Pioneer KEX-20 or KEX-9500. I had to take my Jensen R410 Cassette deck (one of the best at the time) to a tech to have the supplied 5 pin din soldered to the preamp section.
The matching Pioneer components the CD-5/CD-7 preamp equalizer and GM-120 were nice but the Fosgate easily trumped them.
I paid $472 for the PR-2100 back in like '79 - '80 and was the envy of many people back then. I had Jensen Triax II's 6x9's that were bi-ampable they were capable of handling the power in the doors of my truck and the 4" ones in the rear pillars.
The first time I heard a PR-2100 was in a '79 T/A 6.6 ***** Am the guy had a portable Nakamichi 250 cassette deck mounted on the armrest the preamp was mounted in the stock deck location (I loved the long tri-color LED power display) he had the Cerwin Vega CS-15 HED's in the rear deck and in place of the rear seat cushion's he had a pair of Klipsch Heresy's. It was unreal!
The only system at the time that would compete was the Alpine setup with the 7307 cassette, 3011 mini-EQ with the 3001 time delay and a pair of the 3002 50W + 50W power amps.
Nice collection of classics you have there. Another worthy mention from the same era was the Craig V503 which was a 36W x 36W (72W) amp and a cool backlit parametric equalizer with 4 adjustable knobs. Very nice sounding.