The RD 75's will go down to 150hz. So give it a nice 350hz cross for extra good measure.At that rate, why use 6.5 in mid woofers? Skip the mids altogether. Go with a group of 5- 10 inchers. You'll be able to go really low. NHT makes a 10 incher with an FS of 23.5. surely you can find something over in Europe like that.
I believe that Jim Griffin recommends woofer arrays not less than 10 inch.
Marlboro