How's a bottlehead crack measure? THD+N is bad if the noise or distortion is the bad kind, that's being measured, but if it's signal distortion due to the tubes changing sound signature, then it's desirable. I don't know how this is measured or if it even is measured in the THD score, I'd have to bet it is a part of the final percentage, as it's measuring for the accuracy of the signal, along with 2nd and beyond harmonics, both the 2nd and 3rd harmonics are where significant alterations in sound signature can occur that change the sound in a desirable way (if you're after coloring, and not clean, if you want clean, THX exists, I got 2 of em, and hopefully my SW51 tube amp gets here from russia soon, and I can have colored sound, and clean sound side by side). So I guess it comes down to what exactly is the measurement hearing, colored sound distortion, or bad static/noise distortion. The bad kind is why I never picked up a darkvoice (that and the fire hazard of it), was introducing too much of the bad distortion, I want it to register that the perfect 1khz tone was distorting into other frequencies slightly, but not noisey/static distortion/cracking distortion.