Looking for a good small subwoofer...

I agree with REL, but do they have active crossovers?  I would think that the quality of the crossover would be key.  That would most likely require spending a good bit of money on the subwoofer, or external crossover.  The advantage of the REL is the ease of high level inputs.  

Rythmik,REL,or SVS should fit the bill. I run a pair of SVS SB2000 pro in my main system. They work great. I run older HSU’s in my secondary system,but HSU is probably too big for what you’re looking for. Good luck with your search, and let us know what you end up with.

Small is either the SVS micro or the KEF KC62. Only one of Dem has a remote app for easy blending.

Most other suggestions here are inappropriate.

GoldenEar ForceField 30 - 8" Compact Subwoofer - Black $900.00 Add to Cart GoldenEar ForceField 40 - 10" Compact Subwoofer - Black $1,200.00

This combination of art, science, and scoping out the competition is one big reason they have been able to consistently produce so many award-winning products, including mammoth subwoofers, over the years.

Excellent review in Stereophile

hth

There is a pioneer sub modified listed here, $349 . This pioneer sub is superb.i  bought two.