Might want to consider a sub from BIC America:
http://www.lot25.com/bicamvensubs.html

or from Posh:
http://www.poshspeakers.com/Comm/products/835-810.htm

if you set all speakers to 'large' and subwoofer to 'yes', what low pass point does the 950 default to, and can you change that point?

If the speakers are all set to large, then I would expect the sub to *only* get the .1 channel information -- my understanding is that setting a speaker to "large" will disable the crossover for that speaker entirely. (Unless we are talking about the 80Hz crossover on the 6-channel analog input, of course.)

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gonk -- Saloon Links | Pre/Pro Comparison Chart | 950 Review

Don't know about you but I occasionally listen to my rig,not just tweek it.
I use a BFD on the lowpass side which can be used as a Xover with variable user adjustable slope.
My surrounds have very similar roll off characterisctics compared to the center.
I also don't have any major phase anomolies.

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The Hun