Bad idea.
If you're in the USA look on craigslist before ebay - remember power amps can also sometimes be used as boat anchors, and will cost as much as one to ship!
You can often find decent power amps (Mackie, Peavey, Yamaha, Crown, QSC) used at a decent price. I'd pick Gemini DJ products over Behringer stuff any day, esp for amps.
Another thing to check is are the horns in the JBL's blown? Cause if things aren't cutting thru it may be that your high end frequencies are gone.
PS: I have a made in the USA Mackie 808M that does just fine as far as powering both mains and monitors at quite high volumes in small halls.
I was running the monitor amp side with three Yamaha SM15's chained up per side to equal six... I ain't gonna do the math but I would guess that's quite a load. And it was Loud. As. Fuck.
Oh, and another idea: If you can get into the idea of wearing in-ear monitors - Shure SCL3 Sound Isolating Earphones look cheap and decent - or just a decent set of headphones (Closed back or semi closed), then you could run a stereo RCA-to-TRS cable from the "tape/cd" output on the Mackie into a small headphone amp, and get your vox thru there into your ears, with a volume control. I started doing this on smaller stages that wouldn't permit even a small wedge for where my drumkit was set up... It also keeps the stage volume down too.