I searched but did not find any recent discussion on the subject. I did check the Ampeg V-4B manual briefly and it seems the signal path is all tube? The SVT amp has some sort of (silicon) diod between the pre and power amp, if my memory serves me? (The user manual's schematics does not show if things are SS or tube.) What about other tube amps in production, are they really "all tube"? The Traynor YBA 300, for instance, is called "all tube" but uses OP amps in the tone control stages, according to the Service manual.