The Wolfson DACs are audiophile quality and were found in high-end CD players during that time. If I remember correctly, the 4th and 5th gen iPods featured these, but then Apple went with TI for the later generation.
The problem was that all the components downstream of the DAC are low quality, so they weren't showcasing the benefit of having a high end DAC. One company called Red Wine Audio figured out how to mod the DAC and directly connect the signal to the line-out pins. They called this the iMod. You could send your iPod in for the mod, or you could buy modded iPods from them.
The iMod required a special 30-pin > 3.5 mm connector to plug into a headphone amp. So basically, to realize the high end sound of the DAC, you had to 1. Pay for the mod, 2. Buy the special connector, 3. Buy a headphone amp, and 4. (optional) Buy good quality headphones. So you were out a bit of money for this. But hey, that's nothing new for the audiophile world!
I have this setup, and I am still running it today. The headphone jack is unmodified, so it takes the signal from the DAC, feeds it to the low quality iPod amplifier, and I can do a comp by switching my headphones from the iPod jack to my headphone amp. The difference is clear and obvious, even to my ears which I don't consider audiophile. The soundstage difference is immediate. I also did a comp with an unmodded iPod, feeding into the same headphone amp, and I could still tell the difference.
You can read more about it on Head-fi.org, but I'm not sure if people are still doing it today. I don't think RWA is still selling the iMod or the mod service. Instead, newer audiophile grade music players are coming onto the scene.
Edit: Actually, poking around the iflash site that todd originally posted for the SDHC adapter, I found an article describing a user doing the iMod. This is possible if you're good at soldering. There are detailed instructions on head-fi.https://www.iflash.xyz/modding-audiophile-meets-ipod-diymod/
417gb I read the 5.5g iPods had Wolfson DACs. Does this mean anything?