Hi, which Virus desktop do you have? Is it a TI or an older C, B or A?
+4dBu is for pro-audio, while -10dBV is for consumer.
Pro audio uses three-core cables with 1/4" TRS (tip-ring-sleeve) jacks (these 'look' like 1/4" stereo jacks, but are used for a single balanced mono channel instead) or XLR jacks (usually microphone or mixer jacks). The extra core is used to invert noise signals over the cable, effectively cancelling out noise before it gets to the soundcard or mixer or whatever.
Consumer gear uses the usual two-core cable with TS (tip-sleeve) jacks - like 1/4" mono guitar leads, or phono cables and so on. These are more prone to noise.
The Virus TI's sport balanced outputs, so ideally you would use 3-core cables with TRS jacks with the +4dBu setting for best results. Otherwise use -10dBV when using standard 2-core cables with TS jacks.
The Virus A, B, C's have unbalanced outputs, so you would use -10dBV setting with standard TS cables. Alternatively you can solder up some
pseudo-balanced cables for cleaner signals over an unbalanced setup, but that's another topic.
That said, you can use the +4dBu setting when using standard consumer cables for an extra 12dB boost, but in this case the noise floor will also be boosted by the same amount.