I've had really good experience & sound with Mellow Tone Kits. Easy builds & their build instructions are posted on YouTube videos. They also give you all the specs & measurements for their equipment. Now I've only built 2 of their preamps..one in full kit form & the other just using the PCB boards they sell separately for a good price..though they did go up in price this year. Their boards are very versatile so you don't have to stick to the schematic they share.
I've listened to their GU50 8watt monoblocks recently & was very impressed.. especially for 8watts. I thought it sounded great. It's like $2k for the full kit though. I will add that I really like these guys at Mellow Tone/ValvesnMore. Super cool.
I've built the original ST-70 & find it ok.. good amp for the money. When you part them out all new & spend under $1k it's just a good deal for a tube amp. A buddy has the Tubes4Hifi 12au7 ST-70 & it seems solid & an upgrade to the original..just a little different. I built their most recent SP13 preamp which is good, I prefer the MTK's but they have limited inputs & no remote in comparison. I wanted to try the SP14 & their ST70 but my experience with them wasn't stellar so I've kinda put them on the back burner. Skunkie designs just did a build video for the Tubes4hifi amp that kinda got me interested again. I have some chassis & transformers from the ST70 laying around.
I really like the Van Alstine Ultimate PCB ST70. I use a sub so I cut/roll off the bass on the Alstine amp around 50hz or so. I've built a couple of them at this point. Finishing another one up right now with transformers from Heyboer & Pacific Transformer. The OPT's from Pacific are cheap on price so I'm very curious how they will sound when complete. The owner insists that I'll be happy..we will see. First time I've used non-Dynaco iron.
Pete Millet has some amp designs I'd like to try. He has some 50watt monoblocks that I just seen posted recently..though I'm not sure if he sells the PCB's for them on his eBay page. He has some amp projects on Wauwatosa Tube Factory WTFamps.com..i believe he still sells PCB's for some of those projects.
Edcor Amps/Kits has links to DIY designs.
Transcendent Sound sells full kits
Analog Ethos has very nice looking low powered kits.
Specimen Products. Chicago based has an expensive SET & a couple other amp kits
No personal dealings with those I listed..I just have them saved in my phone from when I was probably searching for kits/designs.
PCB's & schematic designs are not kits though obviously. Like others have mentioned there are not a lot of full kits out there for sale. I'm hoping that if anyone knows of any that exists to please mention them in this thread. Skunkie Designs has done a few low powered amp builds on her channel & posts all the parts needed. I wish I had the tools & skills to build the amp chassis/enclosure. If anyone knows of a place or person that will fabricate/cut out top plates for a fair price please mention it.
At the moment I'm using a solid state class D Hypex Nilai DIY amp. High power extremely easy build. No soldering involved. I've paired it with a non-kit Icon Audio LA4 mkiii tube preamp that I did the Signature upgrade on. In my scenario I find the Nilai kit to perform & sound fantastic for the price.