The stock (standard) resistors are 33K. Other values supplied with the kit are 80.6K, 182K, and 360K - providing an extra 6, 12, and 18dB reduction. The previous owner may have done this substitution on one or more inputs.
The switched level controls have a smaller range than the usual volume controls - that's one of the compromises that makes it affordable. Consequently, the best setup would have the normal listening level 2, 3, or 4 clicks below the maximum for maximum flexibility. That is, as Grainger suggests, about 3:00. Three clicks margin gives you an extra 9dB for the rare occasions you might need it - that's usually more than enough.