...the actual SP-3 boxes are pretty decent and would be worthy candidates for speaker/driver upgrades...for example if you installed an Eminence Delta pro 15A the mid definition would be pretty intense, for the compression driver upgrade Parts Express has a inexpensive yet amazing 1" screw on driver
Here's the compression driver
http://www.parts-express.com/pe/show...umber=294-2806
and heres the Eminence speaker mentioned
http://www.eminence.com/speakers/spe...=Delta_Pro_15A
Of course doing a retrofit like this will cost a few bucks with the added issue of heavy and bulky cabs, however if the retrofit was indeed done and you had a dedicated sub (or didn't attempt to force lows into them) you'd end up with an extremely loud yet amazingly clean pa speaker that exhibits a pretty flat response.