It's a question of loudness. A lot of loud peaks aren't noticeable to the human ear, headphones or no, so unless it's a CD that's been compressed excessively in the final mastering stages, you will have an inclination to turn up the volume, and all the peaks will cause ugly distortion on the 3790's outputs. I only tested with a 45 ohm pair, but this is the common impedence rating, unless it's strictly pro.
You won't be able to get them loud enough, I'm positive. It seemed like the bass was weaker than, say on a professional amplifier or mixer console, so the realtek amp is just a poor design contructed for one purpose: To make something audible that conserves battery power. You're going to need something that outputs a hotter signal, and a USB headset or nice laptop soundcard will do the trick.
There's a slightly more portable speaker solution I've seen that's popular with the competitors of IPOD, the Creative Travelsound: http://www.ferrago.com/story/563
They use titanium drivers instead of aluminum core like the Altec Lansing XT1's, so sound should be pretty decent. But the power supply must be bought seperately from Creative, and it's not USB sound, so you're stuck with the sub-par headphone amp to provide the signal. I also don't like the idea of the drivers being exposed in a portable environment. Too easy to damage.