I have a 5w Bugera combo with an 8" speaker that I think is really holding it back, so I want to build a cabinet for it.

Would it be practical to build a open/closed back cabinet, that when closed would be tuned for C standard tuning, but would sound fine when open for standard or 1/2 step down?
Is there a big sonic difference between a 10" and 12" Jensen Alnico speaker?
Will using two speakers instead of one diminish or enhance the super saturated, broken up tube sound I get from the cranked 5w amp?