I think the 6SN7 can work with this amp with the change of a couple resistors.

There aren’t many commercially available OTL headphone amps around for a lower price. There’s a Darkvoice and a Little Dot made in China, but the parts are inferior as is the assembly work. I looked at sourcing all the parts separately and it wasn’t that much less.

The SEX kit I bought is transformer coupled so I can use my lower impedance headphones (AKG). My gut feels is they’ll be the ones I’ll like for guitar, but we’ll see. Sennheiser has a nice rich, warm tone that’s great for listening to music but idk if that color is what I’ll want with guitar tone.