Re: Input vs. Output level
-10dB is usually the LINE level that consumist elecronics use to link different devices.
If you have some CD, sound micro-chain, your PC' sound card or whatever else (I think, even keyboards), those usually have their outputs at LINE level.
And, as said, this is usually (but not always) an unbalanced signal.
A guitar or bass will work at INSTRUMENT level and, usually, they need to be pre-amplified.
+4dB is often seen in Studio Gear, like mixing desks, compressors, EQ, AD/DA converters, etc.
This is the STUDIO level of signal that is mainly used for studio outboard gear and, it's balanced.
It's not just a matter of what signal level (voltage) but, impedance has a big rol in those kind of level types.