That a Panasonic or Pioneer avr?
Don't get a DT sub would be my recommendation, they not only overstate extension particularly, they're just not very good subs IMO and particularly in light of your fairly large room volume, think they'd just be insufficient. No reason to match brand except for aesthetics in any case. I'd think about at least dual 12"s and would prefer at least dual 15"s; if it has to be small and the budget in that range check out the
SVS SB-1000 or
SB-2000 or
maybe the RSL Speedwoofer 10S.
I'd keep up with your L/R and go with the CS9040 as a better match unless planning to get rid of the 9020s. The center has most of the action in movies anyways and I wouldn't think of it as overkill, many recommend the center to be the most capable of the front three (or have identical front three ideally).
Why an Auralex pad? You can make something just as effective and far cheaper; no magic in Auralex unless you like their looks or something. May not even have any benefit, I'd try subs without first.