I'm not an audiophile so i can't offer you a detailed description of the sound the HIFIMAN Sundara produces. Besides, there's more than enough reviews of these cans on the interwebs anyway, such as this excellent review by the respected Joshua Valour...
And a follow-up review, again by Joshua...
As i said, i'm not an audiophile, but i have taken a small step in that direction with the purchase of an external AMP/DAC combo and the $350 USD Sundara. Except for the Sundara, the only half decent cans i have is the open-back QPAD QH-85 headset which caters to gamers (it's got a mic) and so they are the only ones i can compare the Sundara to ... and what i have to say isn't entirely pretty.
The Sundara sounds great. It's an open-back, planar magnetic type with very thin and responsive diaphragms that produce a well-rounded sound without over or under-doing any aspect of it. The bass, mids and highs are all well defined in my opinion. The build quality seems really good; contrary to what Joshua said, they don't feel cheap in any way to me. However, compared to my $94 QPAD's, that's about where the pluses end.
My QPAD QH-85 is significantly lighter and more comfortable than the Sundara. While they also have a somewhat thin and short cable, at least it acts like a 'normal' cable that obeys gravity whereas the Sundara cable is like a god damned slinky! It may well be a better cable as far as the internals, but the sheathing, which feels like silicone, is like a coiled rattlesnake poised to bite me somewhere where i'd rather not be bitten. I tried running it through my hands while exerting some pressure on it to heat it slightly and get it to straighten out, but that didn't help a whole lot. Hopefully it will straighten itself out in time, else i may purchase a different cable.
I thought the Sundara frame design, with a suspended headband that molds to your head, would be more comfortable than the QPAD design, however it's really not, largely due to the extra heft of the unit. The clamp pressure is also a too high for my taste, but perhaps this will feel better once the ear pads break-in. Problem is, i couldn't wait for that to happen. The Sundara ear pads are kind of shallow which is nice because it positions the drivers close to the ears, but they are so close that, with the added clamp pressure, the top of my ears were getting a bit sore from contact with the material just inside the ear cups. I wanted to stretch out the frame a bit to reduce the clamp pressure, but the head band makes that difficult because it prevents flexing the frame far enough to open it up. It is doable however and the phones do feel somewhat better after i figured out how to do it (hold the unit in both hands near the drivers and open the headband as far as it will go, then with your index and middle fingers, press down on the top of the frame).
Another niggle i have with the Sundara is that the hinge for the driver housings isn't loose enough which causes the ear pads to not conform to the sides of your bean as well as they could. I put a tiny bit of lubricant on the hinge points but it didn't really help. Again, this is something that may heal itself in time.
So while i'm very happy with the sound, i expected more in the comfort department for my $350 clams. And a better cable too, though the length of it is perfect for me.
In the end, the question is whether the extra $256 bucks on top of the QPAD's equates to a $256 dollar better experience and, from my perspective, the answer is no, it don't, but such is the case in the audiophile world where one can spend thousands on a set of cans to get an incrementally better sound. That said, i'm not sorry i bought them and i'm sure they will feel more comfortable once the pads break-in.