Here's a snippet from a pm from Steve when I was worried my fx loop was faulty (turned out to be a grounding issue with a rehearsal spaces extension lead)
"The only difference between having the FX loop engaged or bypassed is there will be a very small amount of extra top end included when the loop is active. I did that to compensate for the usual loss through guitar cables. This can appear to the ear as a very slight difference in volume."
So I guess the loop itself is buffered to a point?