The advantage of a second subwoofer is that it can help smooth and even out bass peaks and nulls.
^This. Some people have found that adding multiple subwoofers negates the need for parametric EQ down low. In other words, they smooth the peaks and fill in the nulls. You would, of course, need to take the time to properly place them. I'll be helping a buddy place three dual opposed DIY 18" LMS5400 subwoofers (yes, thats six 18" LMS drivers), then adjusting for delay, phase, individual crossover and adding EQ if necessary. It will probably take all day, but the results will be striking to say the least.
Saturn, that SVS EQ could be amazing. Currently there isn't anything on the market that adjusts for phase as thoroughly as I'd like. The Behringer DCX2496 does a decent job, but I know it could be better. Perhaps the SVS unit will be the answer?
Just give us your bank routing and account #, Saturn, and we'll be sure the money moves.