[Wayne A. Pflughaupt]

I want to use a large power amp to drive the sub-ins.

What does that mean? Amps drive speakers. And what kind of a “large amp” doesn’t have its own gain controls?

And what do you need the level controller to do? That will make the difference on what kind you need. If all you need to do is attenuate, a common stereo pre amp will do. If you need to boost it, that's another issue altogether.

Regards,
Wayne