Crutchfield sells or used to sell a triway or trimode speaker-level crossover that takes a stereo (2 channel) output and converts it to stereo high pass channels and a mono low pass channel. If this device can be used with any amp, then Ish can use it with his amp to bridge the output for a subwoofer. It shouldn't matter that the high pass channels are unused.

I think public enemy means that an amplifier is a lot more complicated than a battery, light bulb or speaker, and will likely be damaged if the outputs are wired together in series or parallel. So don't try it

Sixto
91 300SE
... definitely not an electrical engineer or auto sound pro