The TA-30 is similarly quirky, in that channel 1 has Master Volume post-FX loop and channel 2 has Master volume pre-FX loop. Meaning, if you're running 4cm on channel 2 and you accidentally pull the Mesa preamp out of the equation, you're (very suddenly) getting the entire power amp blowing your hair back. (In my case, all that's required is for my HX Stomp to run out of battery power.
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If you're dealing with a similar issue, then diming the Master Volumes on the MkVII should (approximately) normalize the preamp outputs across all three channels.