Xen,
We have nearly identical set ups. I have the 950/770 combo. I just got my 20-39 PC+ yesterday.
I started with the RF3's crossed over at 40hz. I tried it and bumped it up to 60hz.
I'm still playing with it.
SV recommends setting the gain to 1/2 - 3/4 and adjusting from there.
-15db does sound a little low.
If the sub is near the fronts, I believe the phase should be at zero. Without trying to integrate 2 subs, the phase is negligible.
I'm still trying to get it dialed in, but I'll let you know what works for me.
I have the hum, too. Contact SVS...others have said they have a hum, and SV will troubleshoot the problem and help you fix it.