remington,
Just some questions to ponder and which might help with your design. Anyway, this is a diy thread.
I see that you're cutting the tweeter at 1.5khz (2-6.8uf). Your 1st order filter is not steep that your tweeter has output even below 1khz. Is the audio nirvana or mark audio driver not able to reach those frequencies well? There are more frequencies overlapping your 2 drivers in this cut-off which I think is not good. My unsolicited advice is to cut the tweeter higher depending on the output of the fullrange drivers. Btw, have you tried measuring the output without the tweeters?
Also, the vifa is driven separately by an amplifier so you have no issues with gain. Maybe a waveguide is better used than a horn if the issue is directivity. Just my 2cents.
Larry