It would require some homework, and researching what speakers are around: BUT if you measure the inside of the box you have, including the dimensions of any/all ports, you may find a suitable replacement speaker that will sound alright with that cab. It is still a bit of a gamble. Your best bet really would be either buying or building, IMO. Do you have WinISD? Why do you feel the speaker in there now is "crappy"? It may well be a low quality speaker, but it is still just as likely to sound better than any random speaker as it is likely to be easily improved upon. Ya just gotta follow DA RULEZ to get good results with bass cabs.