Welcome to the forum.

Perhaps you could give us a little more to go on; are either the sub or the 561 new to you or has the problem developed recently? Do you know that the sub is working fine & it only has a problem in conjunction with the 561? What else is in your system? Is the cable in question a known quantity?

Cinema: Meridian 271, 3 x DSP640 (behind screen), 4 x DSP320 (in-ceiling), 4 x DSP3100 (surrounds), 2 x AC12 (interface). 2 x Perlisten D12s subs, Lyngdorf MP40 processor. DT 145" 2.35 AT screen. Sony VW870 projector & S-T lens, Lumagen Radiance Pro.
Lounge: Roon, NUC, Kef LS50 Wireless + 2 x REL Gibraltar-2 subs

Meridian owner since 1998