rack vs stomp box

[bongomania]

Just an observation: that may be something that can be a difference? Is there a +4 dbu and a -10 dbu difference between stomp boxes and rack units? And if there is, would a Line Level Shifter fix this issue?

Line level, instrument level, mic level explained
Effects loops, and pedals vs. rackmount
Yes you can use a level shifter to solve these issues, but be aware that the differences between various pedals, rack items, and amps can be a wider variety than most shifters are meant to address. Imagine a shifter as a choice between two settings, while the actual gear may need input or output that is not either of those particular settings. It is a matter of luck whether you get good results.