Subwoofer Suggestion

I have a very budget system consisting of a Dynaco ST-35 and a pair of Klipsch RF-3 speakers. I recently switched to the Dynaco from a solid state solution, and while I really enjoy the new sound much better, I do feel that the bass is a little weak. Not to mention that the Klipsch are not so good below 40Hz.

I would like to augment my system with a little subwoofer action. I do not want the sub to stand out like a sore thumb, but want it to blend as nicely as possible.

The system is not really for home cinema, but more for music listening. I listen to all types of music, including rock, techno, classical, and jazz.

Can anyone recommend a sub that will just give me that little bit extra in the low bass department. Something I can buy used on this site for a few hundred would be great.

Thanks