Attractive distortion. Not low order, high order, harmonic, non-harmonic, intermodulation, temporal, mental, imaginary, or drug induced distortion in particular ... but the accumulation of and resulting signal waveform convolutions created by the stacking of all distortion products. Very different in different platforms. Maybe the DSP guys could offer some interesting thoughts, too. I'll bet there's even an analogy to be made between how we create a 3D movie image and a 3D audio image where the human brain performs the function of the 3D glasses in the audio case and distortion creates the blurred image. Food for thought........ Question: AC or DC heater filament current? An objective answer: Use DC to avoid sending a waveform into the filament and eliminate IMD and mains hum. A subjective answer: (See Craig Uthus) "AC provides a perfect electron cloud inside the output tube ... creating an incredibly transparent and deep soundstage." [Note: he does this with painstaking care using a 46 kHz sine wave generator, isolation transformers, in the drive circuit that I'm sure he developed objectively. ] Yes, I do think the AC heated DHTs do exhibit these attractive qualities to a greater degree than their DC based counterparts... in my limited experience. OK, it's all bullshit... but you asked for ideas. Also, where's @peef ? Curious if he'd jump in. Baz.