Ok..... So I managed to hook it all up quickly tonight and it all seems to work as predicted.

I have my Engl amp running dry with all pre gain fx
My FRFR wedge running wet with all pre and post gain fx and cab sims....

And it sounds HUUUUUGE!!!!!!
On side note I found I could place fx before the loop on clean sounds and select Stereo on the outputs block in the signal chain, thereby getting a stereo rig, cross delays, washing phasers and vertigo inducing rotary speaker sims......

Very happy bunny, I'll post some pics of it in the room, and I'm test driving it in anger on Sunday at pracco, so I'll let you all know what the deal is there, my main issues are going to be balancing the outputs I think from one patch to another...........

Oh yeah just realised I can still send both XLR outs to a desk for live use, WITH cab sims.....fun..