For one thing, not all that much comes out of the rears anyway. If you've a Sound Pressure Meter and have calibrated the test tone correctly, it will all be correct no matter what you hear or don't hear out of those speakers, the balance is correct. Chadnliz is correct about overkill on those rears. Artificially boosting the rears won't give you the true theater effect designed into the soundtracks of the movies.
Now this might not be the answer you want but......
There is a guy here selling b&w 805 set with a center. Buy them if you can and never look back. Your center especially needs to be upgraded and complimentary to your Fronts, it isn't and probably very lacking. Matching the rears and having a 'voiced' system is ideal! That would be my top recommendation.Your speakers would be killer (your fronts already are) for H/T and any music too. The center is critical for movies as is the sub. If there is anywhere to skimp, its the rears.So if you want to spend less, you can match up the rears nicely with some CDM1's, any version really. Ideally the CDM1 NT series as the tweeters are the Nautilus, like your 805's and would voice fine. A good set can be bought for 700 approx. You can also find CDM Cnt's (Center) for under 500 too.
Frankly I'd forget about 7.1. Movies are mixed mainly in 5.1, so in my mind I'd much rather just have that PERIOD leaving more money then for the best gear I can afford. If you had a giant room and/or a giant budget, that recomendation might be different. I actually have 7.1 but I wish I had never bothered.
Now, I will say the B&W's love power, 85 watts from a receiver probably isn't enough. An older 5 channel amp for now would be wise move. The processing in your receiver is probably okay, just feed the speakers some POWER. Don't need to go crazy here, I'd find an Anthem MCA 5 at 180 watts per channel and reasonably priced used. Plus they turn on automatically from the signal supplied by the receiver, a big plus if you don't have any triggers on the HK.
Good luck, you really have two great fronts - compliment them and you'll be a happy camper!