Guitar Pedals sound terrible?

Have added a Boss DD3 & Hall of fame 2 Reverb to the send return on my V10 (Just wanted to test as have the pedals anyway).

Am connecting both using just Mono. When Either EXT1 or EXT2 are selected on the Send section and the Send dial is turned there is almost like a slight filter effect when turning and the FX just sound bad (slightly crackly and rough)

Any advice, Just thought I would test see what they were like as had them anyway?

Ive also had this issue on another mixer and just gave up in the end. Pedals are absolutely fine and so are the cables as i user them for my amp and guitar with no issue at all.