4krow

 You are fortunate to have one of these units. Dennis is a fine tech and has helped me several times over the years with my work. So, I am sure that there is a lot of work besides the Gundry circuit, but in itself this circuit provides some EQ that gives the illusion of being further back in the music hall, say row 7 instead of front row in some cases. Mostly the effect improves upon a recording that might be just short of harsh, in your face, or overly bright. That is one takeaway of this circuit. If I knew more, I would say, because there is more to it than this. I have used them in C-9 units and some like it, while others don't bother with it. That is why I made it switchable. 

 Do a search here for Gundry circuit to learn more about this filter.

 I would be interested in knowing what is involved in a rebuild.