Best floorstanding speakers less than $500

Normally, I'd recommend a very efficient, or voltage-loving speaker on stands.
You always have to make trade offs at your price point. Since your amp uses negative feedback and, thus, has good damping ability, I suggest a speaker whose main fault is flabby woofers.
However, the Wharfedale Diamond 9.6's are efficent and present an easly load.
And, since they're 4-way (to help efficiency, probably), the woofer's are, I'm guessing, relatively, flabby (hey, four drivers--you get what you pay for).
So, from a technical stand point, they should, probably, work.
However, will you like the resultant sound? I have no idea.

Don't sweat it. You can get that sound by choosing the right tubes for your amp. Go new Chinese or Russian, or NOS Amperex A-frames, or, maybe NOS German/Siemens and their ilk.
You, already, have an an amp that's not warm. It's "neutral". So, it shouldn't be very hard.
For example, new, reissued Tung Sols, etc.
Ask Kevin Deal at Upscale Audio. He'll set you up right.