Is Slate generating a tiny amount of noise? The hiss could be a side-effect of that getting bitcrushed. Maybe bounce out a second of just Slate and zoom right in vertically to see if there's anything else in there. If this is the case, a noise gate after Slate will knock the noise out.
Agreed, I'm at work on nightshift at the moment however my minds been ticking over and I've been thinking along the same lines,
Yes Slate whatever it does to produce its character is probably inaudible to our ears yet Bit crush is picking it up.
Shame it looks as if I'm just going to have to keep them two plugs apart.
Yeah this doesn't seem like a bug to me. The bitcrusher is crushing the bits so far, so that the noise-floor of the previous plugin is being raised. I'd use a noise-gate before Maul, or after Maul to cut the noise out when no sound is going through the channel.