seek a threashold power amplifier

Component wise, the SA12e and the S1600e are identical.

Pbnaudio, study the STASIS design and you'll see.

Higher bias into class A results in a more sweeter sound also. That is why Nelson preferred the 60 Watt stereo SA3.9e. Relative to the other SA series Thresholds, it was biased higher into class A per watt.

Main issue is heat.

1 horsepower is about 750 watts.

Threshold didn't make a 1 horsepower Class AB amplifier in the 80s. They did make one in the late 80s/early 90s. It was called the S/1600e. It used the same chassis as the top of the line Class A SA/12e but was biased to operate in AB.

From my book...
Rated output: 1 horsepower @ 8ohms @ <0.15% distortion
Bandwidth: flat @ DC to -3dB @ 100KHz
Slews rate: 100 volts/uS
Output current capability: 35 continuous/200 peak amperes
Power: Two 1.2KWatt transformers & 216,000uF capacitance
Input Impedance: 50K ohms RCA/600 ohms XLR
Output Impedance: 0.1 ohm
Gain: +32.6dB
Weight: 130 lbs each

Personally, I prefer the higher biased class A designs at lower wattages. YMMV.

I've seen a few SA12e amps for sale over the past 5 years. Never a S1600e. But if you found the SA/12e amps, I think you'd be hard pressed to give up their class A sound just to convert them to 1 hp under class A/AB bias. I would think it more likely for an owner to go the other way. Perhaps that's why there aren't that many S/1600e amps out there.