Another possiblity might be to use the amplifier's XLR input. (I'm assuming the sub has a phono - RCA - output).
Make up an RCA-to-XLR cable that is wired RCA-signal to XLR-phase, RCA-earth to XLR-antiphase, and XLR-earth to the cable shielding. That way the cable screening remains connected to earth (ground) at the amplifier end, but there's no possiblity of a ground loop.
1) You'd probably have to get a cable made specially, as phono-to-XLR cables and adapters aren't usually wired this way.
2) Your signal will be reduced by 6dB.