Subs for Treo CT

I’m looking for some advice and help from folks with more experience and knowledge than me.  Just bought a pair of Treo CT’s and am blown away.  I have one Rel T9i sub hooked up, and it’s a noticeable improvement, even as good as these speakers are.  I have a small 10 x 14 space.  Thinking about two subs, so the question is:  a pair of T9i’s, V2W’s, or Rel s/510’s?  I don’t understand how an external cross over works, or whether I’d need one with any or all of these pairs, so that could add additional cost I know.  I also understand diminishing returns as the cost goes up.  Does it even make sense to consider the bigger Rels, since the cost at that point is just a bit less than the Quattro’s?  My front end is exclusively streaming, and I listen at moderate volume.  I don’t know if this is enough info or not—This level of equipment is something new for me.  Thanks for any help!