Welcome to AK, and thanks for sharing your experiences.
Anyone who is attempting to set up a receiver/amp and is not completely certain about what they are doing with the DC Offset and Bias adjustments should read enough of EchoWars sticky:
http://www.audiokarma.org/forums/index.php?threads/amplifier-distortion-dc-offset-and-you.5634/ . It is kinda large (nearly 4500 posts), but start at the beginning, hit the high points, then go back and read the parts that are relevant to what you are trying to do.
Trimmer pots in many of the "vintage" receivers are OLD and may not have been designed to last this long. My suggestion is to replace them with multi-turn Bourne style trimmers before you make any attempts to fire it up or make these adjustments. Just make sure that you know which way to install them so that CW or CCW turns will increase the bias in accordance with the instructions in your Service Manual (SM).
Here is my understanding of bias (and I'm sure I'll be corrected as necessary):
Transistors exhibit distortion whenever they transition between the OFF & ON states. The distortion begins as the transistor approaches the transition and starts to diminish as the transition is completed. This is not a 1/0 thing!
Bias is applied to the base of the transistor to ensure that it is always enough above the transition point that it does not get close enough to get into the region of distortion.
You adjust the bias with volume set at zero (max attenuation) so that there is minimum "signal" flowing through the output TRs, AND after the unit has 'warmed up'.
The bias trimmer is adjusted to the point where there is a minimal acceptable voltage applied to the base of an OP TR to turn it on enough to let the described bias current flow through the emitter resistor. This usually indicates that the OP TR has been biased above the transition point to satisfy the manufacturer's specs (when built!).
This is a delicate dance.
The bias depends on the connected circuitry, line & P/S voltages, temperature, and other factors . . . Changes in any of these contributors can affect the bias. Like changing some components or component degradation, letting a unit sit for 6 months (winter/summer), and many others.
I wonder if there is anyone out there who keeps small digital ammeters permanently attached to their receiver/amp so they can easily check these levels with a glance.
Clear as mudd?