Discrete voltage regulator
The power supply clearly has symmetrical positive and negative output rails, which only works if the two grounds are connected together. I suspect the grounds are actually connected together at the chassis, and the reason for keeping them separate elsewhere is to avoid a ground loop between the power amp and sensitive input circuitry.

Q214 drives the JFET which switches the input signal into the amp. The JFET turns off when negative voltage is applied its Gate, so the input is enabled when Q214 is off and its Collector voltage rises. 

D201 rectifies AC from the transformer and produces a negative voltage on Q214's Base (more negative than the Emitter which is at B-), so the input is only enabled when mains power is present. This ensures that the amp will go silent as soon as it is switched off. R214 and C215 provide a delay between power on and input enabled, which may stop the amp from producing a 'thump' when it is turned on.