Well, as an EE, or at least a recent EE graduate, I think it's something I can really do well at home.
I have a couple scopes, signal generator, counter, and the like sitting around, and know how to use them.

Output some pure sine waves at various frequencies from the N800, into some appropriate load, and measure the voltage across the load. 20 log10(v/v0) is your friend!

That comment from speculatrix is dead on, of course. The low frequency reproduction of a DAC is perfect. But it is no doubt capacitively coupled, so it will roll off (probably 20dB/dec, not that it matters) at low frequency. I can hook up, say, the comes-with phones, and take some measurements to find the corner frequency.

Regarding the comes-with phones. They killed my ears! Until I took the rubber rings off. Now they fit OK, though the sound is still poor. But having the mic on there means I get to keep them for VoIP, and I'm not going to swap headphones all day. (Does that switch do anything, anyway?) I think, when I have too much time, I shall hack in a 1/8 jack in the mic unit, allowing me to use my Rio earbuds.