That's right, I remember. Depeche is planning on adding another MA Silver Sub to the mix, too.
That sub, as you can see, is a limiting factor.
I am all for you adding extra Silver 300s, but it would be a shame to put them as rears! If you are into multi-channel Audio, they would be much better use at surround rather than rear. Or better, upgrade your fronts to 500s!
XO at 80 is probably fine. Could experiment higher.
Sub is the weakest link. Placement and quality of the unit itself.