I got my HD600 last week for USD250 + shipping (probably free in the USA) so I was wondering the same. Then I realised it doesn't matter if some new phones are announced in May (you bet there will, Senn's 50th anniversary, Munich audio show ...). To produce better SQ for the same amount sounds impossible to me. If something slightly better comes out at 450USD, it's still almost double the current going price new.
This is absolutely correct. With the current pricing, I would bet that the HD-600 and HD-650 are the best deal in audiophile grade headphones right now. I don't think any other headphone is going to get you into sound this good for the $275-$300 that you pay for an HD-600 or HD-650. My understanding is that basically all of the "summit-fi" headphones have flaws that these don't. These reveal detail and even flaws in recordings, but they do so without making the music unbearable to listen to. Sure, you will hear that a recording is bad or that you have a crappy MP3 file, but it wont destroy it. The HD-600 and HD-650 are not ideal when played without an amp, but they do not sound horrible either. The better your equipment, the better they will sound. Additionally, because they sound good right out of the box, you don't have to struggle with matching amps and dacs to make them not sound bad.
My experience with the HiFiMan HE-400 was not that way at all. It sounded horrible out of the box. People were telling me how to fix the flawed headphones by getting different earpads, buying a different amplifier, or even listening to different music (The reality is that I tried them with many genres and there was no genre that impressed me). I'm sorry, but if I have to switch genres, earpads, and amplifiers to even be able to use my headphones then to me that means the headphone isn't very good to begin with. That isn't the case with these Sennheisers. There is no genre with which they simply do not work. They may be better at some than others, but they still sound good with everything.
I also haven't heard anyone say that they are unbearable with any quality DAC/Amp combo. They might sound BETTER with some, but they at least sound good with basically any quality equipment. They sound good right out of the source, they sound even better with a decent DAC/Amp, and I'd imagine they sound absolutely amazing when paired with the best possible equipment. I bet a lot of people end up coming back to one or both of these headphones even if they move on to "better" headphones, simply because of how well rounded they are. I hear that many of the the summit-fi headphones can turn much of your music collection into unusable trash. I think it's hard to beat the refined sound produced by this design. I find that my HD-600's are absolutely flawless for gaming, movies, and TV shows (I don't have cable television, so I use my computer for all of this). I'm not convinced that there are all that many headphones which can sound as natural as the HD-600's do when watching movies and TV shows. Also, the HD-600 and HD-650 are extremely comfortable headphones for long listening sessions. I can wear my HD-600's for 4 to 6 hours and suffer ZERO discomfort. The ear pads are breathable and the driver housings are open, so the air circulates in there and you don't sweat.
Basically, I can't really find a major flaw with the design, and now that you can get either one for less than $300 there isn't really anything else that can touch the overall package they deliver. It makes no sense for most people to spend double the price for something that doesn't really improve on them that much and probably adds significant flaws in other areas (Comfort, treble spikes, picky with amps/dacs, too revealing, etc). I'm sure there are good headphones out there for more, but with these priced like they are it doesn't make a lot of sense.
If you guys noticed, the HD-700 also went down big time. It was as low as $550 new the other day on Amazon. That used to be a $1000 headphone. The only one that hasn't changed much is the HD-800. I think they may be getting ready to do an update to their lineup.