I run some group 27 batteries from costco that are 115ah each. I paid $75 each for them. I'm running a syn4, kicker zx350.4, and kicker 1000.1 and can play all day most of the time. I killed the batteries twice last year playing it loud for about 8 hours. If you have the space, I would go with 3 of those batteries.
You want to wire the batteries in parallel. Wiring them in series would make it 24v for 2 batteries of 36v for 3 batteries.
I have a guest 2611a that is 5amps per channel. I don't have any complaints about it, but if I had to do it over again, I would probably go with a promariner prosport20+. Several of my friends have them and are really happy with them.