Mix and match subs?

I’d go with the bigger second sub.  As mentioned above, matching exactly isn’t as much of an issue with subs, but that added bass down to 20Hz or so could add some real magic in an expanded soundstage and better imaging along with better depth.  Again, I’d try a PB1000 Pro because it’s literally free to demo at home and the value is very high.  Best of luck in whatever you decide, but I think you’re really gonna enjoy having two subs no matter what you choose. 

Unless you’re hell bent on its looks I think you’re nuts for considering the SF sub at $3k.  First, SF is not a premier subwoofer company (as opposed to Paradigm that makes some of the best subs extant IMHO), and the Gravis lll only goes down to 25Hz (-6dB), which probably is no better than your Paradigm.  And BTW, it pisses me off SF doesn’t publish a -3dB spec, which inhibits direct comparison with others that do and only makes me think they’re not proud of the number.  Second,, it’s a sealed sub that’s a different design from your x10 that’s ported, and I can’t imagine that makes it easier to integrate the two. 

Gotta tell ya, to me another x10 makes a lot more sense on every level except looks.  Then again, given you may be looking for some added depth I think an even better (and cheaper) option might be an SVS PB1000 Pro.  Like your x10 it’s a ported design, but the 1000 would get you down to 19Hz (-3dB) and costs only $599.  Plus, and this is a biggie, SVS offers a generous and true risk-free trial period that includes shipping both ways, and they have excellent customer service and would probably be a big help in integrating your subs.  Either way, just adding a second sub is an excellent idea, and that alone is going to yield huge benefits to the quality of bass you’re getting.  Ok, that’s all I got.  Hope it helps, and best of luck.