I can attest to the fact that running a pre through the processor loop does improve the sound of the processor.. I saw the same thing with my LS-26 and Meridian 861.
Processor input
I have a Audioresearch Ref 3 for testing, but no user manual. I'm comparing it against my own preamp and would like to connect it to the processor input of the Ref 3. The thing is that I can't find out how the volume of the processor input is adjusted. Now it seems like it is about at the 2/3 point of the scale, and I see no reason not having it on maximum.