Hey man, your STA-2000 looks like it's coming along great so far. I had to do a shaft replacement on mine too. I used a barrel adjuster from a bicycle brake for mine. A lot like what you designed. And mine's working great. Mine was in pretty rough shape when I got it. Missing most of the knobs and stuff. I went to my local radioshack and picked up some universal knobs. I had to mod the shafts a bit to make them fit perfect, but once I did, it looked great. I also lost the dual concentric bass/treble function, but I just centered them out and didn't worry about it.
It's one of the best sounding receivers I've ever heard. While it was rated at 75wpc, it's pretty common for 3rd party bench test to show the range of 100wpc. I have mine powering my Mach Ones. Truly a match made in heaven. I picked up both the receiver and the Machs for $30. With some work, amazing sounding setup. Even for what they cost new, they shouldn't have sounded that good. lol