Thoughts on having 2 different sized subs in same room

I'd sure add it and try to keep the crossover very low on both to avoid localization. I think 80Hz is for the birds...and tiny little main speakers. Yours are not tiny. Good luck; I think it can be integrated.

IME, using two different subwoofer as stereo pair is not ideal and very hard to integrate well.

My suggestion is connect the SVS SB12-NSD to ZPre3 preamp and position the sub on the front.

Use the high level (speaker level) connect the Audio Source SW15 to the Crimson 275 output (so you can use small gauge speaker wires and don't need long interconnect cables) and move the sub for best bass response in the room, it could be in a corner, front wall, side wall or rear wall. 

I would think different sub sizes would be perferred as the sweet spot of each would allow you to support a wider frequency range

When it comes to subs, low frequencies make so many large bass modes and cancellation areas that more than anything else it pays to simply have a lot of them. Because more subs in more different locations adds up to more small modes spread around more and this winds up sounding a lot smoother, faster, and more articulate. Regardless of what subs are used.

Take a look at mine.

 Five subs of 3 different kinds. One powered, two ported, two sealed. Each on a different wall and/or different distance from corners. This way each one has a different set of modes and they all add up to incredibly smooth powerful deep bass. 

As long as you keep the crossover below 80 and avoid symmetry (do NOT follow advice on timing, stereo, etc!) then all you have to do to get superb bass is adjust levels. Integration is a canard. When the subs are crossed over low and distributed asymmetrically and levels are right they will blend seamlessly. Regardless of what type of sub they are.