What's the input circuitry like?

At this price, I'd expect the amp to be single-ended, which is fine, as I want balanced inputs for a long cable run and don't need a fully differential design. The intended use is for surround speakers, co-located with the speakers.

However, does it simply take one input and shield, or both inputs, referencing one to signal ground (which would require signal ground and chassis ground to be isolated)?

The former would lose all benefits of a balanced cable run.

Preamp is an Outlaw 990.