Yeah, I had some Fluance back when I wasn't sure if I was actually into this sort of thing.
I have Onix Rockets now for my speakers with an SVS sub.
I recently got a 3.1 setup for my parents off craigslist and "tested" it for them for several days.  $750 got my a pair of M22s (with the Axiom 24" stands), a VP150, an Onix ULW-10 sub, and an Onkyo 602 receiver.
https://mywebspace.wisc.edu/spmclaughlin/web/Random/Axiom_Front.JPG
For your crossover question, yeah... pretty much.  It's not an immediate 100% transition though.  The crossover you set is the point at which the sub 
starts to take over.  If you set the crossover at 100hz, your speakers will still be contributing to the sound output below that point, but the sub will gradually take over.
I used an 80hz crossover between the M22s and my SVS 20-39 PC+ and it worked quite well.
As long as you get a good performing sub, it shouldn't really matter if it's from the same brand as these speakers you're looking at.  Aesthetics might be the primary concern about matching 
 
Any of these speakers should be a substantial upgrade in terms of sound quality compared to the Fluance stuff.  The Fluance sets are good at being loud and sounding reasonably good for what you pay for them.  Sound should be a lot more natural and realistic sounding with these choices.
As for 6-ohm vs 8-ohm, you should be ok.  That HK 635 should have no problems driving any of these options.
In comparing the Axioms and Rockets, they each had their strengths and weaknesses.  My rockets had significantly better bass reproduction as expected and have a warm sound that I really like.
The Axioms were very nice as well and had a brighter sound that really opened up the higher frequencies which sounded great for some material.  In other cases, I really missed the warmth that the rockets provided.
So anyway... personal preference 
 
I have not heard the SVS speakers or the X-LS from AV123, but from what I understand the SVS's would be fairly neutral with the X-LS's being warmer.
As I stated before, it would be great if you could try out a few options in your home and do some testing to see what appeals to you personally.
As for the VP100 vs. the VP150, I only used the VP150 but it seems like the goal for that design is to get a wide dispersion going from the speaker.  Maybe the Axiom forum would be a good place to get info on that based on your own situation.