Dr. Z Mini Z
The Dr Z Mini-Z uses the same circuit as the original, but this time they added a built-in attenuator to dial back the output. This allows you to use it as sort of a "master volume", yet you still get to keep the tubes running at full plate voltage for that thick sustain and rich tube harmonics that only a tube amp can give when cranked, but at lower volumes.
Now, if you're looking for clean headroom, the MiniZ is probably not going to do it for you. This little amp is meant to be used the same way you would use a old school Fender Champ. Crank it up, and let it rip! Some cleaner tones can be had by backing down the volume on both the amp and your guitar, but let's face it... That's no fun with an amp like this!
With the typical guitar, you'll start to hear it overdrive at around 9 o'clock, so it comes on early. Naturally, it'll be a little later with lower output pickups, and a touch earlier with hot. As you crank the dial it thickens up a bit as the overdrive and gain increases.