The volume cap can't be removed.
How does the volume of the recording affect the results? I didn't match the volumes. Are my results useless?
Here's some better results.
Well, I dusted off my old PC and tried three different PCI sound cards that were lying in a box. I got much better results with the Terratec Aureon Sky 5.1 (which wasn't the most expensive of the bunch). I measured a DR of about 92 dB. The iPod LOD is actually the winner with that setup, but all devices fared quite well (see individual results).
And, lo and behold, my wonderful Asus N75SF laptop with "Audio by Bang & Olufsen ICEpower". Seems legit!
Those measurements look a bit strange for the ipod (in headphones), I mean the lobes around the 1kHz sine wave. Were you using it plugged into a power adapter? A smps supply could cause that. They dissapear on the rockbox version which I think is interesting. It would be good to find out why.
I got an EMU 0204 USB and made new measurements. I think I need a better stereo splitter cable for loaded tests, because the stereo crosstalk values are suspiciously high. I even managed to squeeze out 7 more decibels in one case just by wiggling the jack :-/