More power is probably going to be difficult in that form factor without forced air cooling.

A lower noise floor could be achieved by ratcheting up the feedback, which has its own set of problems.

An ideal solution for an amp in a "pizza box" form factor would be a digital amp as they are substantially more efficient (~90%) and produce a lot less heat and require less input power for a given amount of power output. The downside is that I haven't seen any "bargain priced" digital amps yet that I'd put in the audiophile arena in terms of sonic quality. But it's only a (short) matter of time.

Cheers,