David, I have 1 S300 and 2 S300 series 2. Both have basically the same specs. The amps run at 12 amps continuous with peak being 30 amps; this is straight from the Threshold brochure and manual. They say that it was "burned-in" to a 2-ohm load (for 3 days), and tested a dead short prior to being shipped. They claim that it will run any load as long as it doesnt go into thermal shutdown, but Threshold doesn't supply any numbers or claims of power beyond an 8-ohm load, which is 150 watts. They also say the power supply will deliver 1400 watts for minutes at a time. I don't know exactly what the "optical" part is of optical biasing is, but I can tell you the biasing is a point when the amp goes from pure class A to A/B. This can be adjusted, but I don't know how to do it.
One person you should contact, who is quite helpful and was an engineer for Threshold at the time of this amps production, who also works, modifies and repairs these amps, is Jon Soderberg at Vintage Amp Repair. Here is the web address, http://www.rsvlonline.net/vintageamp/ I have had him repair one of my amps and upgrade the caps, and reset the biasing and the difference was night and day! He is a great guy to talk with and knows his Thresholds.
One person you should contact, who is quite helpful and was an engineer for Threshold at the time of this amps production, who also works, modifies and repairs these amps, is Jon Soderberg at Vintage Amp Repair. Here is the web address, http://www.rsvlonline.net/vintageamp/ I have had him repair one of my amps and upgrade the caps, and reset the biasing and the difference was night and day! He is a great guy to talk with and knows his Thresholds.