I went through everything, and am using -11dB on the receiver right now. I'm getting a combined 77-78 dB's with all 3 subs using my RS meter from the listening position.
The flattest response before EQ comes with my 600 at 180 phase. Interesting enough, I remember Ian and Alan saying placing the 500 and 600's on 80hz versus bypass brings about a flatter response around the crossover point. I am seeing that on the SMS graph.