The layout of the VoxBox mic pre/compressor section is a little wacky, but workable and actually quite cool once you figure it out.
The input control is actually an attenuator that is the first thing the mic signal sees after the input transformer. Think of it as a continuously variable mic pad that goes from no attenuation at the fully clockwise position, to infinite attenuation at the fully counterclockwise position.
The next link in the signal chain is the compressor. That's right, it's BEFORE the mic preamp. I'm not certain, but I think the compression control just controls the amount of level sent to the compressor side chain and the threshold is actually fixed.
Then comes the actual mic preamp which has stepped gain from 40-50dB. It comes after the compressor, and if I'm remembering this right (I don't have access to my VoxBoxes right now) the level coming out of it has no effect on the compressor. You can sorta think of it as a gain make-up stage.
The way I usually set up the thing is to first get a ballpark level without any compression. Part of this process involves setting the stepped gain not by how much level I need, but by what setting sounds best, because the tone of the preamp changes subtly as you go from 40 to 50. If I've got too much level at a setting of say 50dB, I just turn down the input control until everything's good. After that, I just dial in the amount of compression I need with the compression control and I'm good to go.
There's just two things that I always run into when using the VoxBox:
1. I usually have to set my initial levels higher than normal if I'm using the compressor, so that after the compressor attenuates the signal, my levels are where I want them.
2. On quiet sources or quiet mics, I sometimes don't have quite enough gain to do what I want and have to forgo the VB compressor and patch something else into the chain. In this case I often abandon the VB altogether in favor of a mic pre with more gain.
