Do I Need a DI Box?

Okay, all metal. I didn't know. There's still no need for earthing. Reading your OP again, I'm pretty sure the problem is circuit noise. Maybe the PSU is unregulated. Check with the maker, anyway. Is there a gain or level control?

Thanks to all for your input. I've kind of sorted out the issue. I turned the input level on the RC300 up a lot which allowed me to turn down the amp. This helped a lot. I still can't turn the gain up too much on the amp. Maybe to 3 out of ten, but that's okay by me cause I play a pretty clean tone. I plugged headphones in to the RC 300 and they were clear and clean.

Ken
 
Here's something to try in the diagram below. The effects return would bypass the EQ and any preamp stuff. I think the effects stage could still be used. Although I noted a TRS type cable into the Stereo Aux In, the stereo is summed, so a TS type cable could be used out of the RC300 mainouts. Not sure if this will help reduce the hum or not, but are just some other options to try. I think you were headed in the right direction with re-adjusting the levels.

Edit...... I just looked at the specs for the Loudbox Artist. The input impedance for the 1/4" of the combo jacks is 5Mohms. Not sure if running the RC300 into such a high impedance might have something going on with susceptibility to picking up hum. The other inputs I suggested trying are much lower and would be a better match for the output of the RC300.
 
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