Re: Amp test Bogner/Mesa/Marshall/Diezel/Legacy/Elmwood/ Cornford
Actually, no. The tone does change as you crank the amp, for several reasons. For one thing, as the power tubes begin to compress, they "shave off" the spikes on the distorted preamp signal. The tone gets fatter and sustain increases, the effect of your tone control settings diminishes. Then at some point the output tubes begin to distort, and that adds its own flavor to the sound. The second reason is the sag. As you crank the amp, the plate voltage on the power tubes drops, further increasing compression and sustain, because input voltage swing remains the same, and there's less of an output voltage range (AKA headroom) available. Finally, there's speaker response changes wrt volume, but from what I've read, those contribute to this effect the least with modern speakers (unless you have a greenback and count on the cone distortion).
I feel this is why people don't like Carvin Legacy. The amp was designed to be run with master volume at 10 all the time. Folks buy it, set it to 8 o clock and it "doesn't have enough gain". Of course it doesn't - power tubes don't even begin to compress at that volume.
Both the compression and the sag can be replicated at lower volumes very precisely using power scaling, but none of the mass produced amps (that I know of) have it.