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...and then they'd sell you the replacement tubes from a cabinet in the tube tester stand.
Not according to Toole's blind comparison tests, trained and untrained listeners do in fact prefer speakers that measure flat in an anechoic chamber with smooth directivity.
Balanced response is what we hear when we go to a philharmonic concert or listen to the breeze blowing through the trees in Sequoia National Park. Colored is what you hear when you put on ear muffs or stick your head in a tin can. 'nuff said.
Not sure what you mean by a flat sound. If what I think you mean is a "neutral" or transparent playback, it should not be a sound at all. Indeed it should be true high fidelity reproducing what is actually on the recording. It would only sound flat if the recording has a flat sound. Contrast that to a coloured sound which is a departure from high fidelity as the playback chain is becoming part of the music production rather than a silent witness. Sure a coloured sound like the "tube sound" may appeal to some for its euphonic character (and that is very subjective, some like it others don't) it has its downsides by colouring all music the same way and thus denying the listener from hearing the true sound of the actual recording.