Do you happen to have an SPL meter? I'd be curious to know how loud that song would be at -10dB from your listening point. I'd guess well over 100dB, but it'd be just that, guessing. If it was, man, that'd be some heavy duty listening at the listening point in your picture.
Also to note, I've found that getting to that dB level on any mid to budget level receiver is going to start producing some distortion. I have the same model receiver (AVR-886, same as the 2106) in my secondary setup with much smaller bookshelfs. An external amp would help w/distortion, particularly w/the 4 Ohm M80s.
It will be interesting to hear your feedback in listening to the 603s... -TD