I removed the Yamaha plate amps from my system and replaced them with a Behringer A500. There must have been some serious EQ going on in the plate amps.
The three graphs below are:
1 Both subs with EQ
2 Both subs (with EQ) and both mains
3 Both subs without EQ
If you compare these to the graphs in my first post you will see the massive hump centred at around 16hz is now gone, although I used the BFD to bring it back slightly.
I was surprised by the response in the third graph with no EQ. I only really used the BFD to add a house curve.
2subs+eq.GIF mains+subs+eq.GIF sub.GIF