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!