GL4800 versions of below ARE backwardly compatible.
MONO I/P
STEREO I/P
GROUP 1-4
GROUP 5-8
GROUP L-R
MONO PCB
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GL4000 versions of below ARE NOT forwardly compatible.
MONO I/P (gc ground and eq differences. gc ground difference would negate GL4800 mix buss noise improvements)
STEREO I/P (gc ground difference. gc ground difference would negate GL4800 mix amp noise improvements)
GROUP 1-4 (old mix amp)
GROUP 5-8 (old mix amp)
GROUP L-R (old mix amp)
MONO PCB (would require modification)
PCBs not mentioned here are compatible between models.