The Marshall JVM seems to be one of the few current amps still correctly designed to work with effects.
The FX Loop is cleverly configurable as series/parallel, running @ -10dBV (that's -7,78 dBu) or +4dBu, feeding & accepting the correct voltages to/from a (every) pedal (-10) and true line level (+4) processors (rack only as of this day), perfectly matching their input headroom. The secondary loop is placed between preamp/power amp and correctly designed for line level ONLY devices.... putting an instrument level break there would be dummy. Run your pedal in the first loop, configured as parallel, -10dBV and kill dry sound in the pedal.
Are you having any issue or is it just a pre_buying question?