I posted some impressions on the STAX thread from memory, and I also brought the stax set to the melbourne meet, so quiet a number of people got to try it out. [RANT] One thing that that did annoy me was people putting the SR-404s on top of the SRM1 amp, which was very hot after being on for so long, and thus the glue on the earpads started going soft and the earpads shifted position. After some time I sort of managed to get them right again, but still, I had repeatedly warned people several times. Might look into installing a new set of pads soon even though they still sit over the ear ok. Oh well, thats life. What can you do.
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But anyway, might as well jot down some more thoughts since people are interested.

My current stax set-up includes a Zero DAC, modified SRM1 MK1 (ie So its pro-bias) and the SR-404, obviously. My past headphone expriences has included headphone such as various Beyerdynamics, Ultrasones and AKGs. I currently own Sennheiser HD650s.
I think the first thing to describe about the SR-404 is that its very airy sounding - Listening to various female vocal tracks (Asian and Western), the SR-404 is very "transparent", so to speak. In comparison to the HD650, the soundstage of the stax is wider. Diana kralls' voice was hair-tingling, and whilst overall it may not be as musical as the 650, I think the 404s are more comfortable to listen to for long periods of time. I wonder what a tube amp like the T1 could do for the vocals. Speaking of comfort, the SR-404's headband is really nice and I can have the headphones on for ages without noticing.
Mids to highs is where the Stax excels, and for any sort of jazz or vocal tracks I now use the STAX more than the HD650. I do wish for greater overall resolution as there can be a little bit of grain when listening to very high pitch vocals (e.g. Opera), and in some classical tracks with lots of string instruments it can occasionally feel a little sluggish. I'm not sure whether the amp or the 404s are at fault here as I haven't had a chance to try another stax amp.
One area I do have slight problems with is bass. Maybe the 336+HD650 combo has spoiled my ears a little, but trying some Pink Floyd songs like "money" I just found the stax to be a little bit too cold and clinical for this genre of music. Whilst the vocals were excellently presented, the bass guitar (a vital component of the song) just didn't stand out for me.
I knew what to expect from reading other people's stax impressions, but I was hoping for a little more. Again, it could be an amp/source issue, or maybe my expectations were wrong.
Overall though, I'm really happy with the 404 (thanks krmathis), and they're definitely keepers (until I get Omegas. Maybe

).