Is this a humming loop or not?

Hello to all who read this.
For a long time I have a question again in which I hope someone can help me.

I have my PC (jack, unbalanced) and my Nintendo Switch (jack, unbalanced) mixed with a Tascam US-800 and now hear a significant hum. The PC is grounded and the switch is not. I connected the two devices with an adapter to 4 XLR inputs (mono) of the US 800.
In addition, the humming (almost) fix, if I adjust the volume just right.

I would be glad if someone could help me and tell me if a cheap (not so expensive) DI box is enough to solve the problem.

The US-800 also has a digital SPDIF input. About that you could connect the PC. Maybe that's enough to solve the problem.

Attention: It can also come to ground loops with DI box, if one does not pay attention that also the housing mass is separated. That It is important that the XLR cable has no connection between signal ground and plug housing.