It is obviously a ground-loop problem. There are usually three ways to solve this.
1. Use star grounding. It means plug everything into one power strip.
2. Use cheater plug.
3. If both 1 and 2 failed, you can use a RCA ground-loop isolator like the Jensen SUB-1RR or SUB-2RR low frequency audio isolator. The SUB-1RR is for one channel, the 2RR is for two. They are basically input transformers. Since you are dealing with sub, you shouldn't hear any sound quality degradation.
1. Use star grounding. It means plug everything into one power strip.
2. Use cheater plug.
3. If both 1 and 2 failed, you can use a RCA ground-loop isolator like the Jensen SUB-1RR or SUB-2RR low frequency audio isolator. The SUB-1RR is for one channel, the 2RR is for two. They are basically input transformers. Since you are dealing with sub, you shouldn't hear any sound quality degradation.