Using MOTU's CueMix software, you can toggle +4dB/-10dB on each of the inputs (as well as "Boost"). I've figured out how to do just about everything else from the front panel of my 828mk2, but can't see how to toggle this setting.
Note that MOTU only provides documentation for the 828 Mk3 currently. It appears that the 828 Mk2 manual was available previously in hardcopy, but never in PDF format. I contacted MOTU, and they suggested I use the mk3 manual. Unfortunately, the front panel is different on the Mk2, so the Mk3 manual doesn't help.
My underlying problem is that I own synthesizers that range from balanced +4dB output to unbalanced -10dB. The unbalanced output is very quiet, and I often want to switch over to -10dB mode and possibly turn on "boost" without having to load another app on my computer.