I forgot to follow up on this...

I pulled out my RS SPL meter and my iphone and did a quick comparison. I ran the test tones built in with my denon through my m80's vp150, QS8's and EP500 while holding the iphone in one hand and the SPL meter in the other.

Suprisingly, both were reading the same within 1 db or so.

I did not run a frequency sweep or anything sophisticated.

-David