Ok, so heres the deal.

I have, an Ampeg 810, and i love it to death.

but lets say, i wanted to add.......................... a 2x12 cab, or.... a 1x15.... or 4x12's... or basically, if i wanted to add another stack to my 810, and be able to play through both at the same time, what would i need?

i was talking to my friend today, and he was saying something about an a/b adapter, or something like some sort of box, that i plug into, and it has 1 input, and 2 outputs, the 2 outputs would go to my 2 rigs(Ampeg SVT+Ampeg810 & ________+_______).

could anybody tell me what exactly that box is called? and where i could find them?
Ok. i found out what ABY switches are and stuff, now i have a new question.

Are there any specific ones that you guys know of that are proven to be good?

Do they alter the tone of your bass at all?

Are there any other ways to accomplish what im trying to do?
they're gonna have as much influence on your tone as a normal cable would (it's just some wires and a switch housed in a metal box), bit it has to be properly shielded because otherwise you could get problems with noise. every commercialy available switch would probably work for you.