Yes you're right about the peaks and the math. In practice you can usually run 2 or 3 Systems on a 15 amp line without any trouble so 5 amps seems to be fine most of the time.
I spent a little time banging around the web and on sears.com looking for anything suitable. There might be something out there but I couldn't find it. I did find a few things, but the run-times were counted in minutes not hours.
If you're talking about the wedding ceremony, are we talking just 'spoken word'?
If so, even if you have the B1s hooked up, you're probably not going to be pushing them so you probably won't be drawing much from Amp3. (250 watts).