This is a typical well-used Phase Linear Control Board. Those large 1 Watt Resisters and Zener Diodes were used by Phase Linear to reduce the B+ Rail Voltage (+80VDC for the PL400 and +100VDC for the PL700) down to +20VDC for circuit board control voltages. The result was heat and a lot of it. Because these components are used for both channels, it is unlikely that the heat effects you see could be causing your Left Channel weakness and crackling. Those type of sounds usually point to a failing capacitor or a bad Output Transistor in the Left Channel.
The solution to this problem is a new White Oak Audio Rev G Control Board. Among other upgrades and improvements, the Rev G Control Board uses precision Voltage Regulators for circuit board control voltages. White Oak Audio is here:
https://www.whiteoakaudio.com/
The solution to this problem is a new White Oak Audio Rev G Control Board. Among other upgrades and improvements, the Rev G Control Board uses precision Voltage Regulators for circuit board control voltages. White Oak Audio is here:
https://www.whiteoakaudio.com/