I use to have a Cary SLP-50b that had a similar problem (both R and L channels) and I called Cary Audio about it. They told me to try and open it up first and spray a small amount of Cailube (I actually used WD40)on to the small hinges on the volume knob (inside the unit)and rotate the knob back and forth. They told me that if that doesn't work I might have to send it in and have the whole volume control looked at and possibly replaced. I'm glad it worked with the WD40 because it saved me a bunch. I don't know if this will work with Audible Illusions L-1 pre but just something to think about.
goodluck
Rmml