Some background. My first serious amp was a tube amp — a Golden Tube SE40, if anyone remembers those. I had it for a while but never loved it and moved on to a highly regarded 125 wpc solid state amp that retails for $3000 and which has been the anchor of my system ever since. I've changed everything else many times and its gotten better every time, the latest upgrade being a Don Sachs preamp that I ordered on a self-indulgent whim and which made the biggest difference of any single upgrade until now. I've been so happy with that version of the system that I thought I was basically done.
I ordered the ST120 kit because I was bored and wanted a quarantine project. I built it stock (with the TDR and stepped attenuator), thinking it would be interesting to hear an all tube system for a change and that I'd keep it as a back-up to play around with.
I got it up and running with only a minor hiccup and help from Bob. If anyone is on the fence because of doubts about the difficulty of building the kit, let me say: I'm a total amateur (a philosophy professor with zero training in electronics) and I found the build easy and incredibly rewarding.
As for the sound: The improvement over my previous amp has me wondering how I could possibly have been satisfied with what I had before. Every aspect of performance is better, the main thing being separation of instruments. (I guess I hadn't really known what people meant by that before now.) In triode mode at moderate volume, the bass and treble quality are both significantly improved. A jazz drummer playing with brushes used to sound like an undifferentiated whoosh; now it sounds like art. (I had begun to think that my high-frequency hearing was starting to go and that I wasn't hearing details for that reason. I am incredibly relieved to discover that the problem isn't in my head.) Acoustic bass sounds oaky and musical, no more bland thump.
Bottom line: The ST120 is a superior piece of gear in my system. Hats off to VTA for making it available.