Threshold's

I had a series 2 S/300 ( same era ) and there was a slight upgrade at the time but nothing major. It was also a time that they raised the purchase price . I think the more important thing about either would be how well it was taken care of and the actual condition of either unit.Either will get the job done fine,depending on condition....The "S" series amps were and are considered some of the best amps around for that era....

I've noticed that these amps display class A/AB on the face plate. Does anyone know how many watts are offered in class A before they go into class AB. These amps are rated as 250 watts per channel into 8 Ohms, how much power per channel do they produce into 4 Ohms?

Unsound-
I have a very nice copy of the original Threshold manual that covers the S/150 series II through the S/1000 series II and also the Model SA/1 stasis and SA/2 stasis.
If you get a Threshold S/500 let me know and you can copy the manual.
The manual doesnt exactly answer your questions though.
Here's what I found.
Threshold S/series Stasis amplifiers employ pure class A operation in all signal portions of the amplifier except the current sources comprising the current bootstrap section of the output stage,where AB operation is used.In these amplifiers the STASIS section is required to effect domination over the AB characteristics of the current bootstrap section without the use of global feedback loop.

In SA/series Threshold STASIS amplifiers the addition of a new constant current bias circuit has allowed increasing bias of the current bootstrap section to pure class A.This improvement of the current bootstrap section significantly lowers the nonlinearities in its operation prior to coupling with the stasis section.While tis exacts a penalty in the form of cost,efficiency and physical size relative to rated power it results in a instrument whose extraordinay musical accuracy sets new boundries for ther classification "state of the art"

S/500- 250 wpc ( I think it does double down to 4 ohm but doesnt say)
20 trsnsistors per channel
output current-16 continuous.40 peak amperes/channel
Power supply- 1,000 watt toroidal transformer.

hey,give me a holler!