I had the earlier JBL 10's and they did about 70% of my small to medium jobs. For larger crowds, I added a small bass amp and a crossover - that was all I needed. The bi-amping improved the overall sound, added the neccisary headroom and volume, and made loading the van a snap. Three small, light pieces instead of two large, heavy ones.

That way - for tiny jobs...it's was ONE JBL. For larger .... TWO JBL's ..... for the big time - add the bass amp.

Your KB300 can certainly add the fullness, but at what cost? It's HUGE, and VERY heavy.
They are loud as hell, but you need strong shoulders to carry 'em.

As an alternative to the bass amp, try this:
Get a stereo qraphic EQ and run each side into a JBL. Keep one speaker on the floor (near a corner, if possible) and EQ for more bass, and roll OFF the hights a bit. Put the other speaker up on a pole and EQ that one for more highs, aND ROLL off THE BASS A BIT.
(damn caps lok!)

This will made them more efficient, and still give you wide coverage from two locations. An active crossover is best, but this is a good "fix" at a bargain price.