Close to a month since my last impressions & my affection for the 8400 has done nothing but grow.
With over 100 hours on them, they've significantly mellowed out when compared to their initial, "out of the box" bright/shrillness. I'd confidently venture to say the articulate touches of warmth and smoothness (akin to the GR07) they now possess are notable characteristics. They remain highly detailed, crisp, and balanced. Bass has excellent presence and good impact.. all bass frequencies have excellent weight and detail. Mids are clear, uncolored, slightly forward, and smooth. Treble is crisp & airy.. but maintains smoothness at the very upper registers to keep fatigue at a minimum.
With how much they've smoothed out.. they sound very natural to my ears now.. fatigue is not an issue. A member PM'ed me asking which has the more natural, "long term listening" friendly sound the 8400 & the 003.. I responded with the 003.. but doing more A/B'ing, I'd say it's a toss up at this point. Both phones have a lot of similarities, the 003 has a more robust bass response but the 8400 has a more controlled, articulate midrange.. treble is dead even between the two. The 003 sounds a little more spacious and immersive.. the 8400 has more air and reveals slightly more microdetails.
Fortunately, I don't have to choose between the 003 & 8400 cause that would be one agonizing decision.
Here's hoping more people try these and give us their impressions. They're really one of the more under-appreciated & under-recognized phones. For how heavily them seem marketed to the studio professional market, I think they're more than worthy for casual & critical audiophile-level listening.