Got the same problem chain is: guitar - HXone (pre) - Boss SD-1 - JCM800(cranked pre-amp) - Chorus - HXone (post) - dalay - return.
I don't have the problem without the HXone so my quess is that the HXone's noise is just loud enough to get really fking noisy when going to a SD-1 and cranked JCM.
It would be nice if Line6 is responding to this problem :)
It would be awesome to have this function so you never have to worry about the pre/post position in 4 cable method! This confusing anyways, the pre is effect in the efectloop and post is the effect pre effectloop.
It is not possible to assign the fx on/off function to footswitch 4, which is too bad when using an expression pedal with toeswitch (like the Mission Engineering SP-1). Especially when using a wah effect.