Are you connecting the rel to the speaker out from the amp (high level) or sub out (low level)? Is the sub - amp or preamp grounded (using all three prongs on the power cords)?
If low level out make sure your pre-amp and sub are grounded. If High level you may need to get a shielded cable (but dont know if any exist).
I have a B3 and have never had that problem but working with computers for 30+ years I have seen what you are describing with monitors and long cable runs. In all cases there has been a radio transmitter antenna within eyesight. You may need to shield the subs input cable but first I would try moving it around first and listen for any changes.
If low level out make sure your pre-amp and sub are grounded. If High level you may need to get a shielded cable (but dont know if any exist).
I have a B3 and have never had that problem but working with computers for 30+ years I have seen what you are describing with monitors and long cable runs. In all cases there has been a radio transmitter antenna within eyesight. You may need to shield the subs input cable but first I would try moving it around first and listen for any changes.