I'd say the OP is not being taken advantage of on the basis that he did not consult the band prior to making the PA purchase...
As he stated, they did okay the OP's suggestion of taking a slightly higher cut from their better paying gigs. I think, given this particuar situation, that is quite generous of them, especially since thats all the OP asked for.
+1. I call this "After-the-fact" passive-aggressive behavior. Set up a situation on your own w/o consulting anyone or getting their advice, expend your resources(time,money,labor,etc), then ask for help to fix a situation that you created.
To me, that's after-the-fact BS, band or non-band. I see it all the time. It's also pretty easy to call someone on it. Sometimes they get all pissy and whiney about nobody helping them or nobody likes them, putting on the victim routine. See that all the time too. If you fall for their act, it will be a never ending battle for as long as you associate.