Best moderate - small sub for Soliloquy 5.3s

I love my 5.3s (incredible mid-trebble and soundstaging) but am looking for a little more in the low end. This is for two channel listening only.

I may outright upgrade to 6.3s (or another speaker) but am also considering a sub. I'm looking for something not too large nor too expensive.

I'm currently running a YBA Integre (Delta), Analysis Plus Oval 9 shotgun cable, Cary 303/200 CDP, and Signal Cable interconnect (waiting on my PSS Quartets to arrive).