Sorry the DMM is a digital multi-meter. If you don't have one and don't know how to check resistance,skip that suggestion.
The schematic for your preamp which I found on the web shows a standard stereo pot. "potentiometer" 6 pins. One ground, one input and one output per channel.
Take a look at it. It should have a sticker on it somewhere that says 50k or 100k. Those are the most common values anyway. Or if your going to have the work done for you have them check it for you. If it is indeed 50k and you have the physical room for that stepped attenuator it will work and be a nice improvement.
