Best upgradable system components for under $6K

Hello All,

I plan on upgrading my system with components that I can live with in the short term and the long term (hopefully so I don't have to keep upgrading components as I upgrade my speakers).

I currently have an NAD 356BEE integrated and while it is entry level I must admit it sounds very nice. My room is 14'8" by 20'4" and I currently have a pair of Legacy Classics (not Classic HD). My goal is to make my next big purchase on components in the $6k or less range but I would like to make it a once and for all purchase. In other words I'm looking for something I can live with for 15-20 years without worrying about upgrading.

I would prefer to stay solid state and don't mind having an integrated or pairing separates. New or used also doesn't matter as long as it is a great bang for the bucks. My ultimate goal is to one day be driving a pair of Acoustic Zen Crescendo or Vandersteen Quattro speakers. A 200wpc + consideration would be preferred but I’m flexible on that too lol.

Finally my music selection is a bit diverse. I like jazz, soft rock and new age. Some of my favorites are Dave Matthews, John Mayer, the Wallflowers and John Tesh. I value your opinions highly and would love the community's input on the best component upgrades for my $6k budget.