Like all mute switches, it mutes everything except the tuner out.
It is, quite frankly, poorly designed, in that it is a push pull on the master volume, and it is a bit hard to engage the mute without moving the master volume a bit. Strangely enough, the F500, which has the same mute design, has a different knob and a tighter master volume pot, and it works fine.
The mute control on the Momark stuff is more 'standard'... a simple button on/off.
So, if a tuning mute is key for you, there are better options. You can, of course, purchase one of the wonderful new in-line tuners that finally have totally transparent 'true bypass' modes, like the Sonic Turbo Tuner or the Pitch Black, and an amp mute button becomes a non-issue.
Hope that helps, and let me tell you, it is darn nice to be home on a Saturday night for once
