I am seeing some conflicting information around bout this, but doing my own research it looks like in In Multi-Band mode, the “Low” trim pot controls the threshold for the low-frequency band and the “High” trim pot controls the threshold for the high-frequency band. In “Normal” or “TubeSim” mode, only the “Low” trim is active (it adjusts the threshold for the entire signal) and the “High” trim is effectively inactive/not used.
I reached out to EBS to get some clarification, but I haven't heard back yet. I messed around with the trim pots on mine last night and it seems that this info is correct. The "Low" trim definitely appears to affect the overall threshold when in "TubeSim" or "Normal" mode.