I don't think it's necessary to use the same brand of sub as the speakers other than for aesthetics or if you're matching a poor sub with quality speakers. What are you using for a sub now? Are you unhappy with anything about it? I know you mentioned distortion. Since you mentioned the issue with the unmatched center, I think I would replace that first and then worry about the sub. You could also try running a phantom center to see if you prefer that to the mismatched sound.