You probably read Erik Tracy's comparison between the Oppo and the CA640 on CP, but if not...
...close but no cigar. He felt the CA offered finer detail vs a more laid back presentation from the Oppo. That said, Erik prefers the more forward and edgy sound of the RTiAs to LSis and runs his chain through a Yammie AVR. I haven't made any comparisons, but the Oppo running through a dedicated 2 channel pre sounds sweet (and detailed), neutral, not laidback. Also, I personally prefer 2 channel out of the multi-channel DAC, though that just maybe my gear synergy.