I don't think you will get much of an improvement from a center speaker vs. the superone as a center.
I like the recommendation of the sb12-nsd from Amazon. Great value. I have the pb12-nsd in my office with NHT SuperZeros.
Panasonic TCP65VT60, Denon AVR-x6200w
Oppo UDP-203, Dish VIP722
NHT C4 front speakers, NHT C LCR center spkr
NHT Classic 3 surrounds
Velodyne Optimum 12 & DLS-5000R subs