Sounds like you will listen from a fixed position, and I'll assume you mostly listen alone. Hence the Geddes approach probably has rather low value to you.
Then it's a matter of SPL and impact. I would wait until you are in your new room, play around with layout enough, then decide if you need more subwoofer power.
BTW, it might be worth looking into DSP alternatives for the best subwoofer integration and room correction. Something as simple (albeit limited) as the DSSpeaker products (Antimode and such), going all the way to more complex solutions. My only source these days is an optimized computer, so I'm biased towards software solutions, but other options are available.
In my book, with my bias, my 2 cents:
1) get into your new room, optimize layout
2) work on room treatments
3) time-align subwwofer to 20.7 and do room correction maybe only to the subwoofers
4) maybe you need more subwoofer power
cheers