Looking for a good small subwoofer...

A friend bought two smallish horn speakers, wants to beef up the bass with a pair of small subs, (preferably piano black) with built in amps and active crossovers to send line-level high-pass signal back to his EL84 class-A amp to the main  speakers.
These aren't going to get super loud, just fill in some low bass.

Any ideas?

Thanks,
Steve.