See this website link
Bill's email is email@example.com
You could also try Stan Warren of Supermods. If you don't know Stan, he was one of the original founders of PS Audio (the PS stands for Paul and Stan). He later founded Superfon. Stan still builds preamps and would probably be glad to give your preamp the once over. Stan can be reached at (541)344-3696. Best time is 3pm to 7pm pacific time. Stan has done a few digital mods for me.