Another possibility if you really want to play bass with the bottom cab is to turn it into a 2x12. Keep the size the same and get appropriate good speakers for that size. (half as many speakers means you can pay more to get good ones).

The big advantage of this is that eventually you could do a duel amped bass rig running a clean signal to the bass cab and a dirty signal to the top cab. (have a listen to Chris Squire).