There's a feller on AK wondering how and what to set the bias to, either in Hi Z or Lo Z and what the value is.
This is actually a very good question. According to the D500 SM, bias should be set with the impedance switch set to Hi Z.
After I repaired mine, I figured I would "go easy" on the circuitry and select Lo Z which keeps the rail voltage around
+85VDC. In this mode, the D500 is like a "Godzilla PL 400". The 400 has
+80VDC rails.
I set my bias in Lo Z and the amp appears to be happy. There is still a lot of rewiring and other basic "house keeping items" I want to do to it, but for now, it works and it does not have any particular habits.