If I had to guess, I'd say the dip in response is due to opposite side wall reflections. The bass from each sub bounces off the opposite side wall on its way to your listener positions and interferes with itself there. There's not much you can do about this, and in fact, the corner placements you are using actually minimize this effect by putting the subs as far away from those side walls as possible. You will see frequency response issues like these in any small enclosed space.
It is likely possible to correct this and achieve a flatter response using some kind of EQ. If your AVR/processor supports Audyssey MultEQ XT or XT32, it may be able to correct most of this dip. Other auto-EQ system may also do the trick.
Alternatively, you can consider purchasing an external unit that provides PEQ capabilities, which can give you a lot more control than you have with an auto-EQ system. Note that if you decide to use any kind of manual EQ, I highly recommend first investing in a more sophisticated measurement system such as a UMIK-1 microphone and the freely-available computer software REW. Another popular solution is the Dayton Audio OmniMic. Either of these tools can give you a much more detailed picture of your room response and make it easier to explore what's happening at multiple listener locations as well.
Otherwise, you can just accept what you have. A response that's only 6-8 dB down in places isn't that bad at all. Many systems in-room have peaks and nulls > +/- 20 dB.