tight sound and tight $ sub for Silverline monitor

I'm looking for a "cheap" musical sub - if that's not a contradiction in terms. It doesn't have to go super low or super loud - just locking for something tight and tunefull to go with some Silverlines in a bedroom. I'm looking for something to add a little impact and flesh out the bottom end a bit.

There are some relativly inexpensive models from Energy and M&K - are there better alternatives? Thanks.