The OM 10 and 2M Red are both made by Ortofon, the 2M Red has a little higher output. Anyway, if there is 10-15 dB between phono and CD I think you are fine. If you are getting audible distortions you may want to double check the alignment and tracking weight following Ortofon/Pro-ject instructions. I am pretty sure the Marantz and Denon AVRs have the same phono stage in all models as their published specs are identical word for word, so unless your unit is defective (possible but unlikely), returning it for another model isn't likely going to help. Obviously the easiest first step is to try using your BBE through the aux to compare, though I highly doubt that box can do any better.
Now you got me curious enough to want to try my Xpression on the Marantz AVP this weekend. If I do I 'll let you know the volume level difference between CD and phono. I am still guessing around 10 dB, going by memory and feeling at the time. This time I will take measurements with a spl meter.
Edit: I didn't read your post#18 until after, but then now I know I guessed right about the 10dB difference.