Yeah saw your last post " The quickest fix that springs to my mind, do gigs where they have a solid in house PA. The next option would perhaps find a good place to rent a PA system for the night. If you only need it for larger shows perhaps the rental option could balance out.

Past all that...if you want more sound, your gonna need more power. Plus your not just depending on your PA as sound reforcment, as you have little to no sound naturally projecting off stage...your dependence on a PA is almost like that of very large studio mounters.

Outside of that the bass driver is a VERY good idea ( yet your setup now honestly doesn't have the power to push an extra bass driver tossed in the mix ). You would get sonic advantages for sure, just not the volume you wish. An option for this ( if your somewhat happy with your JBLs )...add a bass combo amp (200 to 400 watts 15inch speaker for sure ) " You would be able to run a stereo mix through your mixer/current power amp for the JBLs, and the independent bass amp could take a mono source from your mixer.

Beyond that a near complete overhaul of your PA is in order, not to be insulting. Just the needs you have stated bring me to that. Only other small option I can think to enhance your current setup ( outside of the mixer your about to add ), would be a nice big XX-band graphic EQ that would go between your mixer and power amp. While your able to do EQing with the mixer, the added advantage of a big outboard EQ...you could better control your headroom and you would have more headroom " Also if your running instrument cables from your poweramp to the JBLs, get some thicker cables.

One thing that could make a full PA upgrade not so harsh on you. Your already on the market for a mixer...why not make it a powered mixer. Kill two birds with one stone and fill in the blanks with extra speakers as time goes on.

Sidenote...while your checking out mixers ( powered or not ) make SURE you have XLR left/right outputs, dont want to shoot yourself in the foot with a mixer that only offers 1/4inch outputs. Some basic effects built into the board ( that dont suck completely ) and the option for external effect send/returns are fundamental features you dont what to be without.