An iMod burn-in update, at about the 140-150 hour mark: in summary, I'm really happy!
So here's the setup now:
iMod --> Cardas 6" HPI cable --> RSA Hornet (~115 hours) --> Grado SR60 cans
For this comparison, I went back to the Nano and Sendstation line-out dock. Generally, I've been burning-in the iMod and the Hornet together, listening for a few hours at work every day, and playing them in a drawer overnight at reasonably high volume every night. About once a day I've been doing a detailed comparison of one or two songs, either comparing sources, or comparing amps (Hornet vs HR Micro/DM.) Today's audition was Lyle Lovett's "I've Been to Memphis," from "Joshua Judges Ruth," ripped with ALAC.
The Nano setup is nicely balanced; the Hornet complements it well, leading to a really nice listen. Attack is quick, the sound is full, and the bass is strong. Individual instruments and vocals aren't remarkable though. The sound is nicely balanced, with nice space and soundstage, though the presentation is veiled in comparison to the iMod now.
The iMod has modestly better vocals, and MUCH better separation on the instruments. It also has stronger bass, MUCH better bass quality, and fuller sound. This is particularly obvious on the bass guitar backing. Highs are brighter too. I judge the soundstaging to be equal between setups now. There's nothing at all veiled about the iMod-->Hornet sound.
There are three specific comparisons that highlight the iMod's performance vs the Nano. First, you can easily pick out the lead acoustic guitar, rhythm acoustic guitar, electric bass guitar, piano, and vocals concurrently. Second, listening to the rhythm guitar alone, you can nearly pick out the sound of each string on the iMod -- this is far superior to what the Nano reproduces. Third, the bass guitar has a very pleasing timbre when played by the iMod, but this is lost by the Nano, which just conveys a general baseline rhythm.
Still enjoying the process here. I don't think the burn-in is complete, but the iMod is already surpassing what I had before

Doug