I know the GLD has a butterwoth 24db crossover built in but is there a way to apply this to the mains to use as a crossover or aux for crossover for subs?
Not possible. You can do a LPF with the parametric EQ on an output bus. But is is 12 dB/oct.
Together with the graphic EQ you may achieve 24 dB/oct when the crossover point is carfully chosen… I don’t know how phasing will be.
Greetz
Pascal