^It is my understanding that identical subs equidistant from MLP would not likely benefit from the trouble of gain-matching. The fact that they are within 0.5dB further supports this with a measured result.
As to -8 AVR sub trims, there has been discussion on this topic on the Audyssey thread repeatedly, last not too long ago.
My take: if you don't use AutoOn/Off for your subs, or if this level of signal is sufficient to turn them on and keep them on, then you should be fine at -8. SQ should be unaffected whether it's -8 or -3.5 or 3.5.
Thanks - sort of what I was thinking and why I was asking...also just came across Chris K's comment on the issue on the Audyssey forums...