Why not just buy a switch? After all, that's what you get if you have an amp with 2 sets of switchable terminals.
Let me be clear, you leave the VAC on 24 hours a day or you want your new amp to do this? A good used choice would be the older Belles designed OCM 200 amps, that can be had for a song and sound, reportedly, very decent, almost Class A like. No serious amps will have an A and B speaker switches as you are looking for.