We're different in thinking, so my approach would be different. If I knew I would likely someday get (at least) a 5.1 system I would first buy the best quality system oriented bookshelf I could afford, to use temporarily as L-R mains knowing that at some point I'd repositioning them to surround duty and getting the system's better floorstanding mains. And unless I know those future mains would be pricey true full range speakers (unlikely), I'd have a good sub. What I would cheap out on is the receiver, as that technology changes the fastest.
For example, I was going to buy a pair of new KEF R300 at $1800/pr thinking that I'd add the R700 floorstanding and R200c center in the distant future. As it turned out, I got the R500 floorstanding for the price of the R300. Added R100 bookshelf and R200c center later. I won't be changing any of them because I know now that a separate true 2.0 or (maybe) 2.1 system will be my next move. The 5.1 system I have now will stand as is for "all around" use as it meets all my needs other than perhaps needing a second sub.
Yes, the Paradigm Studio 20 is a nice speaker and probably sounds better to most people than the much less expensive ones you've been looking at. If it were me shelling out that money, I'd be getting a pair of KEF LS50 used or "recertified" from KEF Direct (they pop up there every couple of months.) I don't know where you are seeing "down to 37hz" but Paradigm themselves isn't even claiming that. Seen
HERE. If true, the KEF Q series would not be as good until you get up to the
Q700 floorstanding or better.