Help pick sub locations

[Chrispy]

One trick is to try crossing them over right at 65 hz. That way you get effectively a 4-way bass array at that frequency. May help or may not depending of woofer heights in the mains. If anything, gives more moving diaphragm area to EQ out the null. Perhaps a down-firing sub would help?

Why would down firing make any significant difference compared to different locations?

As to splitting to feed more than the sub pre-outs on the avr, since each one is separately adjustable for level/delay, just make the subs within a single pre-out equidistant from your LP.