I have the same problem.
I have the TF in the amp effects loop and on this particular pre-amp there is no way to adjust the volume of the loop. When I switch the loop in with just the TF in the signal path there is a volume drop. This is with both switches in the 'line' position, which is correct for this set-up. (no combination of switch positions gives unity gain)
It seems like a basic design floor in these pedals. All that is needed is an output level control. Other rack based units that I own have this as standard. It's a real shame as the effects are fantastic. I was going to buy a PF as well, but instead I'm going to return my TF and MF as they are just unusable in a loop setup like this.
Eventide, please put an output volume control on these pedals! (physical knob / menu setting, whichever…just someway to control the output lvl)