No difference as far as i know besides Cosmetics.
Now... if your talking about a Coliseum Lead and a Coliseum 300... there is a HUGH difference.
BUT. The Coliseum Lead and Bass's only difference is the pre amp. If i recall it is voiced differently, and the Lead won't give you as much bass as the Bass... for obvious reasons. I'd bypass the Coliseum Lead Preamp if i were you.

Unless you wanted to try and do the Chris Squire thing.
But the Power amp for the Coliseum Lead and Coliseum Bass ARE the exact same thing.