Manual page 96:
The KEMPER Kone has been designed to have a perfect tone when the cabinet sits on the floor. Whenever you lift a cabinet into free space, it will lose a certain amount of the bass fundamental, as the mirroring of the floor will attenuate it. This effect can be compensated by pressing Bass Boost "which adds 3 dB to the low frequencies."
Can't find anything else in the manual on it but more could be in another section I'm overlooking, nor in specs on the kemper-amps.com page or the kemper US store, but try what BayouTexan wrote above.
Thanks Howard - yes I saw that. Curious what frequencies they refer to (and what Q). I have been boosting around 120hz. BayouTexan mentions really low frequencies (50hz). Let me try that I didn't think of trying such low ones.