Steve,
I think S1 stack looks awesome! I know a couple people (one from the Washington state and the other in Virginia) who owns some S1 but they are holding onto them and they are 120V versions. I guess there are more in the Japan and Asia but I have also seem a few 240V versions in Europe.
Denon was a real inventor in Japanese Hifi and it's a shame they stops playing in the high end field.
I am not sure about the link between the POA-S1 and the M-8000 but Denon was very creative with POA-S1. I think most Accuphase amp shares the MOSFET driver with BJT output as in Denon. Early Denon uses mostly BJT in the amps but later Denon (POA-3000RG and POA-S1) has changed to MOSFET driver with BJT output.
I happen to take notice that early Accuphase amp uses BJT driver and BJT output devices and they later changed to MOSFET driver with BJT outputs (I think starting from the M-2000?). May be this design change happens to Accuphase and Denon around the same time. I am not really sure but it will be interesting.
I am pretty sure about one thing - Denon is expert at Amplifier Design since Day one and the early POA-8000 schematics is a prove.
Kenwood and Accuphase is also legendary in the amplifier design but I am not sure who tops of whom. Now, I am thinking this a bit too hard.