I did what you're talking about w/ no problem. Had 1 cheap sub, added a little better one. Then got a really good sub and used it w/ the better of the first 2. Then got a second really good sub. Each step got better, but there was no problem w/ any of the mis-matches. No problem, but I'm not sure of the benefit either. The better sub would always dominate because I tended to run it at a higher level. I used the weaker sub simply to smooth the room nulls a bit.