Need sub

I can't afford the high end equipment that many can. I purchased a used pair of Polk R40's, have a Denon DRA 37 receiver, and with the R40's only having the 5.5" drivers I feel I need more bass.

I only listen to music in the bedroom. (size 12' x 12')

Would any of you give me some direction for a sub that is not expensive ($200-250) that will fit well with what I have. Any help would be truly be appreciated. I do not want a down firing port. Thanks again. - leg