I have this same problem with my mel9 unit.

I want effect only, so dry=0, effect volume=max.
Unit in bypass gives clean guitar signal very loud. Unit engaged gives the effect sound at around 50-60% of the bypassed volume.
I should not have to add a boost pedal after the unit just to get the effect level to unity. That would be a deal breaker for my signal chain.

Is there an internal trim pot that can increase the effect level to get equal volume between effect on and bypassed signal from the ‘effect’ output jack?

Thanks,
Dave F