I am not a fan of MXR or Boss compressors (too much squeeze, and not enough breathe), but I did like the Marshall Edward compressor.
I use an EH Black Finger which has two tubes and can to the opto thing as well as the newer version of compression. It sounded great on the first two channels of my VH4, Herbert ch one, Einstein ch one, modes 1 and 2, and now Schmidt, ch one.
However, I have it set very low with only a 2.15 compression ratio and unity gain when turned on and turned off. It sounds similar to the original DBX Over Easy compressors of the late 1970's. Very smooth and transparent.
It can do ch 3 of the VH4, if it is set with a low compression ratio and used with a slight boost in volume from the compressor to smooth out the saturation a bit, if desired. I would not use it on ch 3 if I had the gain up above 12 noon.
However, how many of you who own a VH4, use the global volume to add saturation from your power tubes, which is one of the best things I liked about the VH4, and now the Schmidt? The gain is different, and when combined with a touch of preamp gain, adds a great, more open sound to the amp. Most of the compression I have noticed from my VH4 came from the preamp section. Turning down the preamp gain and using more of the power tube gain opened up the voice of the VH4 for me.
I hear a lot of people mentioning the compression factor, but I never had this problem until I got to ch 4, which I very seldom used, unless I wanted to get the audience's attention.