When a loop is set to parallel, the effect in that loop needs to have "kill dry" turned on. That is, only the wet (effected) signal should come out of the pedal's output. Otherwise, you will get a large volume boost.

When a loop is set to parallel, your guitar signal is split - it is sent to the pedal's input and it also is sent to the PBC's internal mixer. The pedal's output is then mixed with back into the original guitar signal. If your pedal's output includes the original guitar signal, then your signal level is getting doubled because two copies of the same signal are going into the mixer. 

By setting the pedal to "Kill Dry" or 100% wet, only the delay repeats or reverb tail are coming out of the pedal. When that's summed with the original guitar signal at the mixer, it sounds correct.

On Strymon pedals, set the "DRYSIG" parameter to "KILL". On Eventide turn "Kill Dry" on.