Running the d6902r in the back will have the upper hand mainly due to the matching tweeters identical to the front comps you are running. The tweeters used on the D series and F series are different so matching them will give a better balanced music environment
Saw a pair of 6902r's for sale used for 28k. Just make sure you don't end up buying fakes becauses there's ton's of them in the market. Watch for the usual indicators discussed here from time to time
Mind if i ask what amp is powering your 2 subs? I assume you also have a separate amp for the speakers? The SS 680.4 is a good amp. How much are you being asked to pay for that? And 307s are by no means SQL subs. Those are pure SPL. If you are going for a SQL setup, might wanna look into some upgrades