Just wanted to share my recent experience with upgrading a Radio Shack 12" subwoofer. I first thought about purchasing a HSU or an Outlaw model, but then realized the enclosure and the 200 watt rms amp of the RS subwoofer could serve well with a new speaker. I found a 12" Dayton dual voice coil model that most closely fit the size of the existing speaker. The Dayton also had impressive specifications that put it well beyond the OEM speaker that RS used.
I changed the design from vented to sealed and added a pound of the nylon acoustic fill material. The new speaker went in with only some minor work required on the enclosure. The result is in my opinion a much better sounding subwoofer. Not as much distortion as with the original speaker and the vented design. Now I can hear down to 25hz instead of 35hz. Much tighter sound as well.
So, if any of you are experiencing upgraditis (I have had three such attacks myself this year), consider doing what I did for the subwoofer.
And yes, for my listening levels, the 200 watt rms amp has plenty of drive even in the sealed enclosure.