Problem: My old Rel Stadium doesn't have a high pass filter, in fact it
has no outputs at all only one XLR input.

Anthony Gallo nucleus micro manual says "If you use a non-Anthony Gallo
sub-woofer that does not have a high pass crossover, then you will need
to use a passive high pass filter."

What sort of filter should I look for or are there other solutions to
the problem?

thanks

N