Ah OK, glad you solved it! I think “RCA” is just the label for the switch, which is meant to change between phono and line level inputs. Phono is intended for turntables, which need extra amplification, so it will route through another preamp. Audio interfaces output at line level, which doesn’t need that extra boost. By having the switch in the middle, it’s possible that neither circuit was connected.
You’ll want to be very careful not to run line level signals into the speaker when it’s set to phono, as this could damage them.
I don’t really know these speakers, but looking at your photo and the photo on the website it looks like you have something connected to the aux input as well. So, I guess you’ll want to switch between RCA and AUX on the remote, depending on what you want to listen to.
As for the hissing/buzzing, it’s hard to say. It could be a lot of things. Some amount of noise is probably unavoidable, especially with relatively lower cost devices, but if it’s really noticeable there is probably that could be improved.
As a troubleshooting step, can you connect headphones to the MiniFuse to see if you hear it through those, too? If not, then the problem must either be the speakers or the connection from the interface to the speakers. Do you get the same hissing when you use the aux input?
Audio cables can pick up noise from power cables, so trying to separate the cables a bit and make sure that they don’t run in parallel can help. You can also get interference from wireless devices.
Another thing to check is your gain staging—meaning the way you have volumes set through the chain of devices, since both the speakers and the audio interface have volume knobs and there might also be software volume controls affecting it. If you have the software playback or the interface turned down really low and the speakers turned up really high, then the speakers are also going to amplify any noise present. Try running with a higher volume out of the interface and a lower one on the speaker to see if it helps.
There might also be a ground loop issue. Is everything connected to the same power socket? If not, try using a power strip/surge protector and plugging all of your audio gear into that. That can sometimes help.