Test Setup: Arta 1.8.5 -> ASIO USB -> Hilo Line-Out 0dbV @ -5dBFS -> DUT -> Hilo Line-In 24dBU @ -3dBFS For reference, that puts the Crack's unloaded gain at ~24dB (15x). This figure did vary slightly based on driver tubes. In just about every plot that means the Crack was swinging 8.7 Vrms (24.5 Vpp). When loaded, gain drops to 20dB. Loaded with 6080 tubes dropped to 17dB. FWIW I did correct for level when AB. I think 5998 tube might sound best because it has the lowest Zout and best drive characteristics. #1: 1k THD #2: 1k (below) #3: Power supply noise floor* #4: HF 19k & 20k IMD #5 Hi-low 250 & 7k IMD #6: 20Hz THD** #7: 1k THD (333 load) #8: Reality Check*** * My build of the amp is surprisingly quiet for a "cheap" DIY tube amp. With the choke in place, I hear no hum or hiss at full pull on the volume knob. CCS and choke combine to do their job quite well. If you're getting hum, also check that input wiring. ** Not shown, but distortion across audioband was constant. OTL's don't have issues with LF linearity. *** I'm not aware of many headphones that require 24Vpp for normal listening levels. This plot shows a more realistic level (0.6 Vrms w/ 333 load) of how much distortion is there in a usage scenario. Even then, keep in mind 0.6Vrms is about 98dBA for something like an HD800. Realistically, you would only see that distortion on large transient peaks. **** Slight differences in distortion and gain were seen with different tubes, but nothing that really correlates with perceived SQ.