Both of them together will most certainly perform better than either individually. I run my EX with a smaller 10" sub, more of a mismatch than your Rumba is, and I can hear an appreciable difference. You'll get a potential smoothing of your frequency response, and a modest bump in output capabilities, so you can run them both lower and get the same output, which can have benefits long term/reliability wise, or run them hot to get more output without as much a load. With multi subs there is some integration to deal with, If you have some form of sub EQ you can better integrate them, but even without, playing with placement and such can work wonders.
I'd bet that used EX is around $400-450. It would be a nice upgrade for the dollar spent in my opinion.