DC filament ground and high voltage ground

[mikeyho]

Currently working on a local Marantz 7 preamp kit with regulated DC filament power supply. In its PCB layout, the amp circuit ground has been mixed with the filament DC ground, in a way that the pre-out ground level is 30mV or so higher than the high voltage PS ground (due to the passing of filament current, judging from the topology.)

I'm aware of the common sense that one should take the amp output ground directly from the PS ground (instead of the PCB ground) to avoid possible "positive feedback" generated by the passing of output stage current through the PCB ground.

Is the above layout design simply a bad practice then, or is it a possible trick to make good sound for such a legend circuit? (To fire up the filament with DC voltage, is it necessary that we isolate the DC ground from the high voltage ground?)

Thank for any comments in advance.