They may rate them in pairs, as I am no experts. However I doknow the defcon III's are composed four of Kicker K 6.2 component speakers. Each individual speaker has 150 watts max power rating.
Maybe, you could run the sub and interior speakers of of the JBL and use the 640 watt amp for the tower speakers. (assuming that the other amp is smaller)
Remember, the tower speakers are to be loud enough for the rider to hear (in theory). I have an amp each for my tower speakers and Sub and run four infinity's off the head unit. I don't really need any more power to the interior speakers because the tower speakers give plenty of volume.
This is how I would do it with your set up, but others may have better insight. BTW if you already have everything wired, buying a new amp online is cheap (relatively speaking).