Is tone stack active or passive?

This is to answer a question in another forum.

Is the tone stack in the Express passive or active. Several places I read that you should be careful in setting the Treble to high as it effects the premap gain downstream and can make the signal less clean, this implies an active tone control. Someone in another forum stated they are passive and do not affect the gain structure.

Many MESA amps use pre-gain EQ and post-gain EQ. Typically the tone stack is pre-gain and the graphic EQ is post-gain. We place the tone stack in the Express before the main preamp gain stages to allow you to choose which of the frequency bands will enter distortion first, and thus influence the flavor of distortion. To set the overall frequencies going to the power amp, we have the graphic EQ post-gain.

Its normal for pre-gain EQ amplifiers to drop signal when the tone controls are all turned down - this is something that is not specific to MESA amps and as the Treble pot is first in the signal path, the way that it is set will affect the controls downstream. The signal path travels from one control to the next, so each component in line sees the signal from the component preceding it.