Desire to try tubes

Used/new Cayin KT88, either the mid size with a  quad of 88's or the big boy with 8 KT88's, 
have caps changed to Mundorf SESGO. For power tubes, Svetlana 6550's, pretubes Tele's.

A tube pre-amp would offer a different sound, for sure, and you’d have to decide if it would be a big plus. A budget going up to $5k for just the pre-amp opens the floodgates for many, many options, plus tube substitutions which can also vary the sound.

Not sure from your post if you're asking about just a pre-amp or a pre-amp AND an amp. $5k would be sufficient for both, but you could really get an amazing pre-amp with $5k as your available budget. You could also get a pre-amp first and then add an amp later.

Rather than suggest specific models, I can tell you what parameters might be worth looking into:

-- matching impedance between preamp and amp: PS Audio has a post on it and you could also email them or search the forums here. See, e.g. https://www.psaudio.com/pauls-posts/mix-and-match-impedance/

-- seeking input from Heresey owners about what they’ve liked.

Good tube providers are also very important to know about -- I’ve had good experiences with Brent Jessee, Jim McShane, and Vivatubes.

I’m sure many others will pile in on this thread with better information, but hopefully this helps get you thinking about some general directions your inquiry might advance.

Thanks, will starting with pre-amp better than tube amp? Do you need both for the best tube sound? 5k would be tops for both if need/desire both. And yes i will check those resources to understand impedence matching did not know about that...

Another option would be to look for a good quality tube integrated amp that would match with your speakers

There is no best tube sound. If you are able, I strongly recommend you audition some tube products to see what kind of flavor you prefer. Some of us like tube amplification that is somewhat  solid state like while others prefer the glow or burnished sound some tube designs can provide.