I had this with my REL sub. Just connect an RCA cable between an unused port on the back of the S300 and the .1/LFE port on the sub. That eliminated the hum for me and is what REL recommends in their manual.
Yeah, that’s exactly what I’ve done. Just used one of the leads from an old RCA cable and went from the S300 to the .1/LFE on the sub. Also, per REL, I made sure the .1/LFE level was set to zero (knob turned all the way down).
Add an extension piece of wire to the REL grounding wire and ground it something else in the equipment chain besides the S300. When I used Class D amps I connected the wire to a grounding lug on the line conditioner. It is also going to make a difference as to whether the amp in the REL is a Class D amp itself or A/B.