Imo, as long as you use a HPF (High Pass Filter) to reduce the low frequency sound (the frequency the subwoofer will be producing) then the 6x9's can compliment the sub very well. Bearing in mind, they should both be in phase with each other (as will all speakers!)
If you do this, you shouldn't (can't?) degrade the audio quality, simply increase the range within which you get accurate audio.
Also, the JBL 6x9s, as with all decent ones, come with co-axial tweeters integrated, so I can't see how they won't reproduce sound properly...
Just what I've picked up over the years, and not a dig at any other information, just a different perspective! Personally though, I'd run with just the 6x9's and the front speakers
Dave