Hi Mark,
One can hook-up as many peamps as desired to the SR6007 either as sources or destinations. How to connect external preamps to the SR6007 depends on what you wish to achieve.
The SR6007 already has its own internal preamps.
You could, if you are not careful, burn the AVRs input circuitry by adding external preamps as "sources". Also, not much improvement in SQ could be gained by external preamps connected to the SR6007's preouts becuase any signals that come out of these have already been processed (or distorted?) by the SR6007s internal preamps.