Wouldn't worry too much about the dB indication of the receiver. Room size contributes to apparent loudness as well. If you room is 400 square feet or larger, you're going to have to get use to adding a few dB's on the display to get good volume.

Even at -10 dB, your unit still isn't delivering an average power of 1 watt to the channels. You have plenty of juice left in the tank.
Just curious, do your speaker volume level settings subtract a few dB's from any of the channels, or has the receiver cut back the gain to the external amp?

I have 3 external amps connected to my RX-V1400. The speaker levels are +/- 3 dB around the "0" setting. Only the center channel is driven by the receiver. I hit 80 dB in room at about -24 dB on my display. Just switching one of my power amps for another model can cause a 6 dB swing. Differences in voltage gain I guess.