Also you can attach a short wire cable to the screw fixings on each device and that may isolate ground the humming issue without spending silly amounts of money.
Also next option is parametric EQ and dial in the frequency at 60Hz and make the bandwidth of the filter narrow and reduce the level or cut down so the humming disappears at loss also with any sound effect that is centred around 60Hz would then be reduced drastically! So try grounding the issue.