Budget subwoofer needed

My Velodyne CT100 sub is on it's last legs and making some weird taps or some such noise now and then, so it looks like it's time to get a replacement subwoofer.

I would like to know from the sub owners out there which subs would be a suitable low cost over achiever to put in the CT's place?

Volume and phase control is important, and a X over would be nice too. line and speaker connections as well, if you please.

This is a bedroom system (14 x 12 x 8) with a Sony 444es reciever, Phase Tech PC 10.5 mains, Phase Tech PC 6.5 rears, and a ghost center ch.

I'd like to spend under $400... yeah, for sure!

Actually, IF such a thing is out there I'd also consider getting a second one or a step up from that for the main HT system's rear ch. which is near done being remodeld and uses all Silverline speakerage (Sonata III; SR 15s; Center stage middle speaker) and a Velodyne DD 15. that room is 19.75 x 13.75 x 8.5 and closed off entirely now.

I'd spend a mite more for that rigs second sub.

Any ideas, or thoughts on either path would be severely appreciated.

Thanks much.