If you've ever used an original URIE FET compressor, you might notice that the input pot on our version is a little hotter. The reason is that the original Allen Bradley pots had a very unique logarithmic curve that is hard to replicate with modern components. Modern parts that can, are VERY expensive and we find they end up scratchy in less than one year. In the past, we used a grey 3-deck pot that was hotter than we would like but did the trick. Recently, we've moved back to our blue Bourns 3-deck pot that is less hot and closer to the original curve. We're very happy with the performance.
However, the revision A's input is hotter by design, a full 6 dB. Some users find they have a hard time getting light compression. Here is a very simple mod you can do to the 3-deck potentiometer that will result in a 6dBish "slug" on the input. You can do this mod on any revision of FET/RACK or FET/500 and it's simple.
Newer FET/RACK PCBs have a R-OPT next to the t-pad for this resistor. If you have a FET/500 or older FET/RACK you can easily still add the mod. Looking from the bottom (rev A Rack pictured) locate the side middle lug that attaches to pin 1 of the input transformer. Solder a 100Ω resistor (anything above 1/8W is fine) from that terminal to the front middle terminal. The front is where the shaft is located.