Noise is unwanted sound in your signal, such a low-level hiss and hum that are inherent to electronic circuitry. By setting up a proper gain-structure you minimize noise by amplifying it as little as possible, but still leaving enough headroom for the loudest peaks in the signal.
The +4 dBu input has a low internal gain and is intended for high-power signal sources such as mixing desks and some audio interfaces. The +4 dBu input is recommended on the conditions that you are both using such a device that is capable of signals in the range of up to 22 dBu, and that you are controlling the volume level by means of the volume control in the 8c itself, i.e. controlled with the lanspeaker app.
In all other circumstances, both domestic and studio, we recommend the use of the -10 dBV input. This input has almost 12 dB higher internal gain than the +4 dBu input. Do you want to control the volume ahead of the 8c? Then turn down the volume in the 8c app as much as possible, without having to clip the source in order to play loud enough. A setting of - 10 dB is often a good starting point. Do you want to control the volume in the 8c's themselves, then turn the source's volume all the way up, just below the point of clipping.
Be careful with turning the gain in lanspeaker up higher than 0 dB. When the gain in lanspeaker is set at 0 dB and there is no input signal, the noise should be at least 118 dB down. With every dB of gain you add, you don't only increase the level of the signal, but also of the noise. If you need more loudness, first see if you can increase the gain earlier in the chain before you increase the level at the last stage.