AVR sub outs...mono or stereo?

pearsall001

I currently have an NAD T765 AVR that has two (2) sub outs. I'm looking into picking up an Anti-Mode 8033 EQ for my dual Velo subs. I contacted the company & they said that I need a "summing" Y adapter (not just a Y adapter) to convert the two signals to mono before I run it into the Anti-Mode. They also said that I could purchase their new version, the 8033S which has two inputs for stereo instead of a single input like the 8033 model. I always thought that a sub signal was mono, but does combining the two signals in theory make it a stereo signal? It doesn't seem like a big deal but I just thought that a standard Y cable would suffice.