We both have Civic's don't we?

Mine is an Accord 4-door.

How big is your amp

Amps, plural, and yes they would clear the underside of the seats. But they're pretty good sized and with my light-colored carpet and "open" interior, they would be rather visible.

The problem isn't whether or not they fit, it's whether or not passengers can fit their feet under the seats. Leg room doesn't stop at the back of the seat, people's feet go under the seats in front of them quite a ways. At least they do in my car.

Sure, someone could sit there comfortably with an amp under the seat, but they wouldn't be able to put their feet as far forward as they could if the amp wasn't present.