Is there anything in that list you aren't particularly happy with?
Sometimes a second subwoofer will help even out the bass frequencies and make things sound even better. Depending on the subwoofer you have right now, it might be a good idea not to have it placed in a corner. That can lead to boomy, slightly bloated bass.
There are newer receivers coming out with Audyssey XT32 auto calibration that have better adjustments for single and dual subwoofer configurations.
One I'm looking at is the Onkyo 818. Hoping for some good reviews.