I've noticed the 2525h I have doesn't seem to like my strymon el capistan pedal very much. The delay sounds really quiet, even with the level turned all the way up. It's sounds fine when used through my badcat so I know the pedal is good.
My line 6 helix sounds absolutely fine with the Marshall so it must just be certain pedals. My old vintage modern, well most of my old Marshalls all had the level switch on the loop. Maybe the 2525h could have benefited from one too.