advice on possible (REL?) sub for PSB Imagine B speakers


I know nothing about subwoofers! I have a couple specific questions and a couple general questions. Here goes.

The context: I recently said goodbye to my old Maggies (1.5), which I loved. I picked up a pair of PSB Imagine B speakers. I'm not sure they're a long term solution but, for a few reasons, I'd like to give them a run for a year or two. I like them! Still, I really miss the Magnepan sound; the fit, literally, just isn't right in my house. For that matter, a floor-standing speaker wouldn't be ideal. Big windows, big view--so perhaps not big speakers. (I love my wife!)

I'm wondering if a subwoofer would open up the sound of my speakers, give it more "air," more warmth and immediacy. I play primarily classical and rock; not surprisingly, it's the classical that I want to enhance. I don't care a whit about rattling windows or big booms. I want something that is "musical" and that gives more presence not only to the lows (organ, double bass) but to the mids (cello, bassoon). As is often said, I don't want to hear the subwoofer when I'm listening. Finally, I want more from these speakers now but I would also want a sub that could work with future speakers.

Almost everything I've read about REL sounds promising. I'm looking at the S3 SHO (still available with a little searching) or the S510. I did read something, somewhere, that the REL subs are designed to complement high-end speakers and not bookshelf speakers. So perhaps it wouldn't be a good fit? I'm open to other subs but REL seems to be saying the right things. Also, smaller is better, space-wise.

Here are my questions:

1) While I realize it might be odd to spend twice as much on a subwoofer (say $2000) as on speakers, is my approach otherwise nonsensical? Should I put to rest the subwoofer idea and save the money for better speakers down the road? Would a well-matched sub have the potential to open up my speakers now? (I do know that placement, etc, is crucial.)

2) More specifically, would the REL work well with the PSBs? Is there a reason they might not be a good fit for bookshelf speakers?

3) More specifically still, the speakers are ported--would I need to block those ports? 

Thank you all so much!


northman
Adding a Rel subwoofer (T/5i) to complement my Kef LS50s was the best decision I've ever made. It completely transformed the musical experience with increased scale and soundstage (width & depth) as well as added warmth. FWIW, the LS50s run full range with no bungs in the ports and I use the recommended High Level (Speakon) inputs on the Rel.  
Tonight is one of those nights. Almost feel like saying get the REL, its The Lord of the Subs, the One Sub to Rule Them All. Because, tired of explaining. But yet not feeling quite that sarcastic. On the fence. So do yourself a favor, type into the search bar, "Swarm subwoofer system" then search here for all posts by Audiokinesis and noble100, then search for "Earl Geddes subwoofer." Then if you still haven’t figured it out, make Sargon proud. Get the REL. It is a magical sub.

Seriously. Even better. REL is the Bose of subs.
Thank you all very much, including Gandalf. I'm now thinking about the T5i or T7i, but I'm still learning, obviously. I'm sure this question is addressed repeatedly out here and elsewhere but I'm going to expose my naiveté and ask it anyway. I understand the principle of matching the sub with the speakers, system, and room, but I don't entirely understand the application. Within reason, why wouldn't I want the best sub I can afford? Can't I just ... turn it down if it's too much for my room? Would a higher-end model (S/510, e.g.) be potentially more difficult to place in my room or to match with my speakers? 

(One challenge for me is that I'd like to buy something that will work well with future speakers, and of course I don't know what those speakers will be. So I'm wondering how to balance what I have with what I might have.)

I live in the mountains of Vermont so I can understand why my $25,000 Subaru is a "better" car here than a $250,000 Ferrari, but I don't know how to apply that to subwoofers.

(And if you're reading, bubinga, where did you audition/buy your Joseph Audio Pulsars? I am enjoying their website but I can't see if they have dealers. They're out of my price range for now, but I'd really like to hear them.)