I couldn't find a switch for my STA-95 either....may be able to take it apart and clean it up. Worth the shot.
http://audiokarma.org/forums/showthread.php?t=463427
If you haven't cleaned the pots and switches adequately, then you need to. I was amazed at the difference it can make. Have a Sansui 9090db that nothing seemed to work. After repeated cleaning and then lubricating everything came back to life except the tuner (entirely different challenge). I've had crackling go away in other receivers, makes a huge difference. Don't expect a simple one shot deal, you may need to spend some time. One thing you will immediately notice is the dials will turn more easily and smoothly. If they are at all stiff, it's definitely long over due.OK, spent a half hour hooking up the STA 2100D and trying to tweak it and I was sadly disappointed.
With all controls set to flat and the Loudness off, the Mach 1's sounded weak and a bit muffled. Turn on the loudness and they sounded bloated and heavy with no sparkle. No amount of short term tweaking with the available controls (receiver and speaker) made them sound right. I was hoping that the RS receiver mated with the RS speakers would be a bit magical, based in part on what I've read in various threads on here.
Reconnected the Mach 1's to the 36 year old Akai AA 1200 next door and once again, they sounded powerful, sweet and alive.
I guess the STA 2100D is just going to have to look pretty on the rack.