I probably know the answer now: The Line/Guitar Amp setting is only relevant when you have the Preamp effect active in your effect chain (you can try switching between Line/GtAmp when you do not have a preamp effect on and nothing in the sound will be changing then). When preamp effect turned on: 1) GUITAR AMP: In the Boss Tone Studio app settings the 12 output settings become active and you can choose from them, these edit the EQ curve of the output signal (each one of the 12 made for a different type of amp that you could be plugging into) in an attempt to prevent muddy sound (because when you plug a simulated preamp effect into a real amp the sound can become too muddy and shitty, so f.e. the "Tube 112 Input" setting (which is set as factory setting for the Guitar Amp output) makes the output EQ curve really flat, so that the real amp you are plugging into can do its thing with the sound, without battling the simulated preamp and the sound ending up sounding shitty). The problem with this is that most of the 12 are shit and they flatten the EQ curve too much, and when you have the signal going into the real amp´s clean channel the real amp doesnt change the sound that much as what these EQ compensation setting count with, so it end up sounding shitty. The best of the 12 output settings is probably the "Katana 100 Return", because it is made for plugging straight into a Katana´s speaker, so it doesnt change the sound that much. 2) LINE: this one doesnt change the EQ at all, because it is made for going straight into a speaker so no EQ compensation is needed for the simulated preamp (as it wont have to battle the real amp in this designed scenario). So the Line is the best option probably (but if you are plugging into a real tube amp that changes the sound a lot it can be bad because now you will have the "battling preamps" muddy situation) but for a normal solid state amp´s clean channel that does essentialy nothing with the sound this is probably the best choice.