Yeah, to be honest I was having problems setting up the aggregate device from day 1, for example I'd click the USB guitar but the inputs wouldn't show up on the inputs section.
Just out of interests and possible future reference, what did deleting Logic's preferences do? because that seemed to do the trick in my opinion. I had tried the same aggregate device setting before but to no avail, everything seemed to go right after that.
Once again big thanks, it's great to finally get this guitar up and running