That switch probably has to do with phono cartridge loading. As such, it should not affect the line section or any other input in the least. That is, if it is properly designed. Sean
Pug what source were you using? If it was the phono input then yes it could / probably would sound different. As Sean suggests, this is a phono stage loading option & would not affect any line level inputs. Look again to see if there is another switch for cartridge loading resistance. You might see values of 47k-Ohm (standard value) & then some much lower values - 100 Ohm, 50 Ohm etc. If you use a moving magnet cartridge then the likely recommended termination is 47k. For moving coils, try the lower values & determine which sound that you most prefer. Experiment with different combo's of capacitance & resistance too.
I agree with Sean. Never had an Adcom but would bet the switch is to match capacitance with your phono cartridge. If you like vinyl look in the cartridge manual for suggested setting. Will likely have to include tone arm wire and cable capacitance in the mix.
Do not change the switch when your pre and amp are on.
Have you checked the Adcom web site for info? That is the only place to go. Manufacturers often have all the manuals and stuff online if you have lost yours.