I just smoke tested both the PL2000 and PL4000 and they both passed. The PL4000 has damaged RCA jacks so I will probably strip the unit down and replace all the RCA jacks with modern gold plated ones. From the quick test I did some of the knobs are a little stiff but all seem to work and all the switches throw without issue. I did not feed signal through them as it was mostly a smoke test and the PL4000 needs some work before it can be properly tested.
As to the mods on the PL2000 it looks like it was modded for karaoke use as it has a vocal attenuation knob and a mic volume knob added. As to the 3rd knob I am not sure yet. I will probably just leave the mods in place as they are historic to the unit and the holes are already in the front panel. I will however try to find tops that match the aesthetics of the unit better than the current ones.
I am currently using the JBL C60 S8R with my Denon AVC-A1SE with 170 watt RMS at 8 ohm, they are 16 ohm speakers and can apparently take up to 150 watt at 16 ohm. I am not sure what the Denon would be rated at when measured at 16 ohm but I suspect it would be under 100 watt.