This is true for muti-channel ANALOG output (ie; lossess audio, DVD-A and SACD), the less trim applied in the Oppo the better. Too much can clip and too little can degrade the sound.
Make sure the Oppo volume is maintained at 100% at all times also.
Some trim can sometimes not be avoided, depending on the analog capability of your receiver (not all have Audessey, etc.)
Have you thought of using the separate analog 2 ch outputs from the Oppo for CD listening? They work great if you have superior analog circuitry in your pre/rec. etc. I know everyone has different tastes, and different setups, it's just a thought.