First off, what is your goal on this?
If you are looking for more sound overall, maybe add speakers. If you only have 2 cans on the tower, why not add another set so you have 4 total. this would increase the sound, and depending on what amps you have you might just be able to hook them into the exsisting amp. Another option is to add an amp so you have more power available to use, the more power you have to use the less you are going to be pushing the limits of the speakers.