Otherwise you are rerouting any returning signal back into the device. A little deliberate feedback is OK for enhancing delays and pitch shifts with delay, but will produce painful results with reverb. Most effects devices now come with ample feedback parameters on-board. If you have a live mike in the room being routed to the studio monitor speakers, you may have given the signal a new path to feedback from through the effects device. I hear a great deal of hiss from the effects channels.
Effects are very sensitive to level.
Some are also very noisy. Chances are, you need to send more signal to the device so the Effects Returns can be lowered. Increase either the sub-master on the channels, the Effects Send Masters, or the Inpu t know on the device itself until the peaks of your sound just hit the red of the Effects Device Input meter. I hear a great deal of distortion from the Effect Return. Again, effect's Input Levels are very sensitive.