Which Budget Friendly Floorstanders Next?

I don't think you are going to like the Focals in all likelihood.  I own older 936s and they are analytical, require really good electronics and cables to get them singing right.  The rich and fullest sound on your list is probably the Vandersteens, but get a model up if you can. good luck

I've always liked the B&W 683.  But once you hear those drivers in a proper enclosure (The CM series) you can never go back.  

I agree with you on the Vandersteens.  I have a pair of 2Ci that are incredibly warm and have stunning low end (always lacked top end though).  I have used them to mix on for years.  And my very first pair of decent speakers were the 1Cs.  I bought them when I was 17 and unfortunately never had proper power for them, which always makes me want to go back to them.

I'd address the preamp first.  The preamp is a very important part of what you ultimately hear, so I'd recommend a good stereo pre first before changing what are already very good speakers.  Best of luck and FWIW. 

Has anyone heard the Revel F35 or Fluance Signature?  Both very cheap, well made speakers.  Can't help but wonder...

Revel F206 used