Here's my experiences on this.
My thought initially was just like most of ours would be: The shorter the signal path, the less sound degradation, the better. Running straight from the source would bypass a whole other piece of equipment, and provided the volume control on the OPPO is mostly lossless, we can only gain signal integrity.
So, I tried it running straight from the OPPO to my amp (Ayre V-5xe, bypassing my K-5xe MP preamp) and the music sounded very sweet and airy, with beautiful sound staging. Problem was, I seemed to be missing low end punch, and highs lost crispness. What was wrong? Why was I losing sonic power?
Spoke with my dealer and he explained that players are not designed to put out enough power to drive an amplifier. They're designed to be fed into a preamp, and the preamp has much more power at its output stage than a player does, and thus is able to drive an amplifier to a superior dynamic range. Dynamic range being exactly what I was missing.
Discussed it with a friend of mine who's a sound engineer, and he explained that one of the differences between pro gear and home gear is that pro amps have a very high input impedance, whereas home amps have a very low input impedance. When an amp has very low input impedance, essentially, the flood gates are wide open and the amp is sucking all the juice out of your input device--hence why home preamps have very high output voltages, it's in order to drive an amp that would have low input impedance.
So unfortunately, yet again theory (or hope) does not always match up to reality, in terms of using a direct path from player to amp. One could still very effectively hook up their OPPO straight into an amp, it just has to have a high input impedance. That would be lovely to hear, and I can't see why a pro amp, or even home amp with high input impedance, wouldn't play just fine with the OPPO by themselves.
Hope this helps,
Omar