From just a couple of reviews of the Oppo going direct into poweramps and using it's own volume control, instead of having a preamp in the system
Excerpts from Hometheater
"Using the BDP-105 as a fully-functioning preamp by disconnecting my reference Parasound Halo JC2BP from the chain, I still got the same level of detail and precision as I repeated my battery of tests. In fact, there was an even greater sense of transparency and openness to the sound. Using the digital volume "knob" via the Oppo remote control was absolutely painless."
Excerpts from TNT Audio
"Things get really interesting when we dispose of the Django preamp altogether and simply plug the OPPO straight into my Unity Audio Rock active speakers. Now the sound really came alive, with even more drive and power than through the Django. Used like this, especially through the balanced connections, the sound is truly wonderful and probably the best digital replay I've had in my system. Going back to the Django, now sounds a little bit blurry, slightly constrained and somewhat less interesting."
As I said before, it's horses for courses. If you have a system that needs no tonal/colouration changes, then going direct will be the most transparent/dynamic way.
Cheers George