not knowing how you will use your mixer and speaker (or two?) limits the advice i can give.
it all depends really on what you intend to do with it. if your going to be playing at high volumes at say a large hall, then unless you have a really good set-up then you'll blow the speaker pretty quickly.
if your just going to use it in the front room, then i'd say you wouldn't need a bass amp.
how many other people will be playing through it at one time?
if you get a good PA set-up and you dont go blasting away too much (in your front room) then you can have one or two guitarists and your bass fairly comfortably, with a vocal mic. if you intend to use keyboards as well you may get distortion problems, or worse.
hope this helped
phill