Press I/O, then go to the top of whatever page is displayed, by pressing Encoder 1 until it reaches the top entry and the cursor (highlighted line) is off.
Then you can rotate Encoder 1 to access the Input/Output in question (Main Out1/2, Phones3/4, Analog Input) and from there you can rotate Encoder 2 to go to the sub-page "Bass/Treble".
From here you can enable/disable it by pressing Encoder 2 to move the cursor down to "B/T enable", then rotate Encoder 2 to select OFF.
It is always a good idea to go to the top of a sub-page (by pressing Encoder 1 repeatedly until cursor disappears) after making a setting as only then you can access the other subpages by rotating Encoder 2.
The menu control is somewhat confusing at first but after a while you realize that it is 100% systematic and once you get it any adjustments are made quickly.