I think it depends on “how good” your amp is, as well as the rest of your system whether you’d experience any sonic benefit. If you’ve got a super high end single ended with no negative feedback, like Japanese Audio Note, French Jadis, VAT or PP McIntosh and the rest of your system is equal, I think you’d like the KR Audio tubes. If a “lesser” amp, like Line Magnetic, or Willsenton, not so much. The last two are not bad amps, but run negative feedback to reduce distortion and mutes some of the second order harmonics which makes tube rolling less impactful.
I don’t know about the KT 88, but I have a Cary Audio 300SEI LX-20 that is designed for the KR 300B XLS which puts 550 volts on the plate with 55w dissipation and 20w output. The Cary will eat normal 300b’s like a six year old eats through Frosted Flakes! The KR XLS in the Cary are fantastic, Deep, tight impactful bass, something normally missing from 300b. The midrange is simply heaven on male and female vocals, acoustic guitar and strings. The treble is clear, clean and very articulate.
I have a Willsenton R800i 845 with Gold Lion 300b in front of Shuguang 845B output tubes. Both significant upgrades over stock Linlai hifi series tubes. Would I swap out the Gold Lions for KR 300b? No, probably not. I don’t think the $$ value equation would be there.
I also have a Dennis Had Inspire KT-88 that came with Gold Lion as stock. It will also run 6L6, EL34, and 6550. The Gold Lion KT-88 is the most balanced, linear and musical tube I’ve heard in the Inspire. It’s a simple SEP design with one 6SN7 or 6SL7 as the signal splitter and driver in front of the output tubes, no negative feedback, and hand built point to point wiring. It was only $2,000. Would I roll in KR KT-88s at $900/pair? Maybe, if I won the lottery!
Bottom line, I’ve had good experiences with the Gold Lion series of power tubes, definitely a fan. Do I think the KR tubes are an upgrade worth the cost? Absolutely! If your pocketbook and system can handle them.