https://www.audiosciencereview.com/...d-crack-headphone-amplifier-kit-review.15714/
While the BottleHead Crack is a one trick pony that can yield good results, if you enjoy how it colours the sound of a high impedance headphone, great. I owned the HD800S and had a very nice La Figaro 339 with amazing tubes and it was a euphoric sound, but using the speaker tap cable and driving the HD800S from the speaker terminals of my NAD M3 which is a solid state amp was everybit as good as the OTL 339 could ever be. Better if you consider that I could also drive a 16ohm headphone equally well.
Really, just get an affordable and well designed SS headphone amp, of which there are several excellent candidates now, and if you love the coloured sound tubes provide you can just use an equalizer to add the colour. OTL amps can sound lovely with the right headphone, not saying they can't, but they are very limited in terms of what they play well with.
Still, I get the magic of the tube glow, they are seductive little things.