Groove Tubes can be used for guitar or hifi. They commission tubes to be built for them. They used to use JJ to make an awesome hifi tube, the E34Ls which Manley used in their Snapper amps until JJ made a bad batch. The tube is still listed in GT catalog, but I don't know if JJ still makes it or if it's any good. It was very powerful and very low distortion, a copy of a GE version of the tube with some hotrod mods, designed by Mitch Margoles, who is a great hifi amp designer. They also sell a Mullard EL34 copy which is higher distortion, intended more for guitar. Groove Tubes is a quality tube seller, but shit happens, especially when you hire JJ. Supposedly GT sent the original GE machines and original GE metal parts material to JJ to make those E34Ls. A JJ version of the tube soon appeared, then the quality fell. Maybe GT is making them somewhere else now, I don't know. But they were so good, I would be tempted to try them again. The price is much lower now. Iirc, there was also a GT 12AX7 tube that generated some excitement in the hifi world a few years back. Rambling...
EL34 is the most famous sunny and fat sounding guitar tube after John Lennon recorded with it, but it's also the most spacious, liquid sounding pentode tube for hifi. If you like space and beautiful tone, EL34 makes a great hifi amp tube. A designer uses the tube specs to his advantage whether his goal is high or low distortion. The brand name doesn't matter, except for quality and reliability, of course.
