Since you're in a bedroom, I think the general consensus to stay with a 10" subwoofer to save space is a good one. To realize the greatest improvement, I recommend a ported sub over the Hsu STF2 that j_garcia recommended. I think you'd notice a significant improvement with either the
RSL Speedwoofer 10S or the
SVS PB-1000. Both reach 20Hz at satisfactory levels, which would offer an instant advantage over your Sony. They'll also play louder at much lower levels of harmonic distortion.
Think of it this way. Would you rather drive on the Interstate in a V6 Honda or in a 3-cylinder Smart car? They'll both reach 70, but one strains with much greater effort than the other.