Message from REL regarding sub connections;
Tank you for your interest in REL Acoustics.
Connecting high level to the Sansui 907 Limited should be fine - To connect a single unit should be as shown in link below -
https://relsupport.zendesk.com/hc/en-us/articles/115004486467-Connecting-a-Single-REL-via-High-Level
To connect a pair of units should be as shown in link below -
https://relsupport.zendesk.com/hc/en-us/articles/115004649048-Connecting-a-Stereo-Pair-of-RELs-to-a-Single-Amplifier
I can see the amp has a GND terminal - Connect the black wires to this GND terminal.
I hope this helps, Please let me know if you have any problems or further questions.
Best,
Ben Farmer
Another members post regarding connection;
"If your amplifier is Class D or Balanced/Differential, then the red and yellow wires will stay the same but the black wires should be left floating, or connected to a chassis ground screw. The Phase switch should be left in the position which gives you the loudest and deepest bass output. Typically with stereo pairs the phase switch will be in the same position for both subs."