As indicated in the manual, from which you were quoting, the negative speaker terminal should absolutely not be used as a ground point for the sub, because the amp has balanced outputs and therefore a signal is present on that terminal. A chassis screw can't be used either, even though the REL manual might suggest that, because in this particular amplifier chassis is isolated from circuit ground. What Pass will probably suggest is a means of rigging up a connection to either the ground sleeve of an rca connector, or to pin 1 of an xlr connector.
Regards,
-- Al
Regards,
-- Al