I can only comment on the HD600 in comparison to the LCD-2 (Jan 2014 production), as I don't own the T1. That said, here's a breakdown of how I perceive the LCD-2 & the HD600:
As one would expected, the tonal balance of the two headphones is simply different. Coming from the HD600 (and the T1 based on what I know of it), the current LCD-2 will initially sound slightly dark. I found it took a bit of time to get used to after being accustomed to the HD600. Once I got acclimated, I actually found the LCD-2 to be the more balanced sounding headphone. Like you, I found the HD600's treble could be a bit too "harsh" with certain recordings (one's choice of amping obviously plays in to this as well...another topic). In my opinion, I find the HD600's treble to be a bit higher in the mix than I prefer. As a result, the LCD-2 is less fatiguing at high volumes. Ultimately, the LCD-2 possesses a sense of fullness that the HD600 can't match; comparatively the HD600 can sound "thin".
Probably one of my favorite aspects of the LCD-2 is how it is very smooth and grainless, while simultaneously being incredibly dynamic. It seems like a bit of a strange dichotomy, but I find it to be one the LCD-2's greatest strengths. Everything sounds effortless and yet it can hit hard when called upon. It is also faster and can gracefully deal with complex passages of music that I've found the HD600 to struggle with. All in all, the LCD-2 is a step up in terms of transparency and dynamics.
A lot of attention has been given to the LCD-2's bass presentation, and it is indeed very impressive, though often for reasons that I didn't anticipate when I purchased the headphone. I would describe it as "reference" sounding, which is rather unsurprising if you look at the virtually straight line response in to the sub-bass frequencies. That is to say, it sounds like it should sound given the recording. It is not a basshead can. However, it does indeed sound a bit bassier than the HD600, due to the slightly higher treble presentation of the HD600 per a given volume level. As I'm sure you're aware, the HD600 also gently rolls off in to the sub-bass, which adds to the perception that the LCD-2 has a stronger bass response.
Both cans are very good in the midrange, but the grainless transparency of the Audeze bests the HD600 here as well. It's very impressive. The treble is obviously a contentious point of the LCD-2's sound signature. In my opinion, it's liquid, smooth and effortless, with a nice sense of sparkle. I do not find myself wishing for more "air". That is to say, I generally find it to be close to perfect. That said, I know folks have different impressions on how they want their sound presented. With my WA7, I do use a Moon Audio Silver Dragon cable with the LCD-2, which brightens it a touch (if you believe in the aftermarket cable kool aid).
I find the soundstage between the two to be very close in terms of width, with a slight edge going to the Audeze by my ear, though that may be in part due to the LCD-2's greater sense of transparency. The sense/perception of depth is better with the LCD-2. Initially I was disappointed in the imaging of the LCD-2 compared to the HD600, as it was a bit "hazy" when I first received the headphone. Moving to the Silver Dragon cable proved to completely remedy this problem and my LCD-2 now sounds very precise with imaging that is on par with if not slightly better than the HD600...close enough that I'd call it a tie.
Comfort seems to be hit or miss for people when it comes to Audeze products. Thankfully they fit my head very well. While heavier than the HD600, I don't find the LCD-2 to be any less comfortable. I've had 6 hr listening sessions with the LCD-2 and wasn't bothered in the least. Clearly this isn't the case for everyone. I would encourage anyone to demo them somewhere before buying for this reason alone.
All in all, I find the LCD-2 to best the HD600 on pretty much every level (unsurprising given the price difference), with the caveat that the tonal balance and comfort isn't everyone's cup of tea. The HD600 is a great headphone for the money and I still enjoy it for certain recordings. All this said, I would recommend demoing a pair of LCD-2's if possible before buying. The new Hifiman HE-560 also might be worth looking in to as well if comfort is a concern. Hope that helps, best of luck.
Edited by JoelT - 4/12/14 at 4:39pm