I did wonder about grounding the 2x4 chassis?
The crossover chassis will be grounded to the same point as all the other analogue signals via the phono plugs. If the PSU is grounding it to a separate point, that will create your ground loop.
Turntables ground to mixer. Mixer connects to pre power, 2x4 sits inbetween with jumper cable with outs to power amp and powered sub.
Any issue there?
Loosen off your phono plugs, you may find they’re making contact with the case and creating additional ground loops.
Start at the power amp and work your way backwards.
@Currid started
London Fixed Gear and Single-Speed is a community of predominantly fixed gear and single-speed cyclists in and around London, UK.
This site is supported almost exclusively by donations. Please consider donating a small amount regularly.
The crossover chassis will be grounded to the same point as all the other analogue signals via the phono plugs. If the PSU is grounding it to a separate point, that will create your ground loop.