This is easy - British sound, $6K budget new/used, for jazz & vintage rock...buy the best pair of Proacs you can find & afford. If you end up staying with solid state, you'll be great. If you end up going tubes, the Proacs will kiss you on the lips.
I've walked a similar path as you...moving up the B&W line, solid state to tubes, exploring new "British sound" frontiers, and I don't think many people would dispute a Proac recommendation at your budget.
(I'd also tell you to look at Wilson Benesch - I have a pair of Arcs and love them just as much as my Proacs, but at your budget it may be harder to find a pair of floor standers. But either way, Proac or WB, I think you'd end up happy, again just from my own experience which sounds similar to yours)
I've walked a similar path as you...moving up the B&W line, solid state to tubes, exploring new "British sound" frontiers, and I don't think many people would dispute a Proac recommendation at your budget.
(I'd also tell you to look at Wilson Benesch - I have a pair of Arcs and love them just as much as my Proacs, but at your budget it may be harder to find a pair of floor standers. But either way, Proac or WB, I think you'd end up happy, again just from my own experience which sounds similar to yours)