I had one of the rack mounts. I bought it in the late 80s. I was running it in front of my JCM800 and it sounded great.
I do agree with the other post it is not a stand alone unit for distortion. It even makes the cleans more chimey when set at a very low drive. I later came to find mine was an unauthorized version. It even said B.K. Butler. I saw a Joe Satriani interview where he stated he used one on Surfing with the alien. If my memeory serves me correctly,the rackmount had Master, gain, mid, treble,bass, contour and bias.