tc electronic BonaFide Buffer

When measuring the frequency response it is perfectly flat all along the spectrum (that would make it suitable for low tuning instruments). But when I plugged my passive guitar into it, very audible loss of treble compared to entering directly 470kOhms input... So I suspect a passive filtering or loading at the input to simulate entering an amp, but I didn't manage to get the info (yet?) fom the manufacturer. So the ">1MOhms input" spec seems a little useless... Some people may like that but that is not what I am looking for.
Also I don't understand the bypass power failure option for something that doesn't work on battery... Wouldn't your entire rig shut down?