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Yes those circular capacitor trimmers don't help. You don't know whether it's increased or decreased until you change it.

Low bias kind of makes sense. It increases the treble (by not erasing it as much) but that can also create a rather brittle sound because it magnifies dropouts due to tape imperfections, dirt on the tape etc. The normal bias current somewhat overbiases the highs which actually helps with this problem. In record, as the tape momentarily lifts slightly away from the head due to dirt getting in the way, this reduces the actual bias to the surface of the tape, which increases the treble a little. It's not perfect but it helps a little.