Thanks all for your responses.

I should have clarified that this is about playing electric guitar, so "preamp into the house system" does not apply. The way my amp is built (it's a clone of the Dumble Overdrive Special) means that the tonal "sweet spot" comes with overdriving the output tubes, not just the preamp tubes. But I'm finding that with 50 watts this just can't be done except at ear-shattering levels (I don't play stadiums).

It's also not financially feasible for me to buy a different amp or speakers. I'll look into wiring the speakers in series instead of parallel.

I welcome more input!