This has been a long and fruitless search and I need to pull a Hail Mary on this, in case there's a glimmer of hope.
 
I hate bright headphones. I have a pair of DT 990 pros and they give me pain and headache every time because of their excessive treble (even though I like them otherwise). So now I realize that the only sound signature I would like is the dark signature that HD 650 (or HD 600) is known for.
 
Since I already have the 990 (which I will continue using with some eq) and should have HD 600 soon, I also need to buy a third pair of closed headphones which I could use casually listening to music on my laptop, through my phone, or make field recordings with. That means they'd have to be low impedance (I plan to use them unapmed) and comfortable enough to be used for extended periods of time. They certainly need to be over the ear headphones, because on-the-ear types hurt my ears within 20 minutes. 
 
With so many headphones around, one would think finding what I want would be trivially easy. But after two days of research, I am beginning to lose hope.
 
First, I thought I found the perfect headphone when I read about the DT 660 being close, over the ear and ideal for classical music (which is all I listen to), then I found that they're almost as bright as the DT 990 pros. Later I narrowed down on the DT 150 and was about to order it, but a last-minute double-check revealed it to be 250ohm, which isn't really suitable for mobile applications. ESW9 was my last disappointment. Some claim it to be almost indistinguishable from HD 650, and I almost had a heart attack when I found out they were on-ear phones...
 
Will the good people on Head Fi help me?

This one's easy!

DT250 (80 ohms, if you want to use them unamped).

cjg888 said:

This one's easy!

DT250 (80 ohms, if you want to use them unamped).

Click to expand...

I had considered these too, but they're just not known to be warm. Some people even find them a bit brighter. Here's from an amazon review, which I am afraid to ignore... Others have expressed similar concerns. 
 
"Fatigue both aural and physical. The treble peak above gets pretty tiring after a few hours. About the same time for me, my outer ears started to ache from the pad pressure. These are not circumaural, they will press on some part of your ear, depending on how you fiddle with them. For an hour, maybe two, no problem. Beyond that, a break was required."

I've found them to fit perfectly and to have a neutral sound signature (250 ohm version), largely comparable to the HD600. If anything, I would criticise them for being a little on the dark and smooth side...

cjg888 said:

 If anything, I would criticise them for being a little on the dark and smooth side...

Click to expand...

Might be the tube amp you're using with them. I plan on using them umaped all the time. That could be a different experience.

They also sound like this on the end of one of my solid state amps.

Good to hear that. Hope they sound the same way unamped.
 
Eager to see what my other options are. 
 
EDIT: Looks like DT250 are not going to cut it either. This is right from the first page of its appreciation thread:
 
"My only issue is that they are not truly circumaural and like the Sony MDR-7506, they partly rest on the ears and not around them. This makes them uncomfortable to use (for me) for extended periods of time."
 
As I mentioned in my first post, I want them completely covering my ears and not rest on them or touch around them.

I have no such problem. In fact, they are extremely comfortable.

At the end of the day, it depends on the size and shape of your ears. These are designed as tools for broadcasters, and comfort and ergonomics were clearly a priority.

I would recommend trying on a pair to see if they fit you...

somi said:

  The Denon D2000, V-Moda M80 are close to HD650 with more bass but decent treble response. Shure SRH440, SRH840, Klipsch Image ONE are also very good portable headphones for casual listening.

Click to expand...

They are portable headphones alright, but none of them really have the smooth treble of HD650. I am very sensitive to treble and that's why it's been so hard to find a really dark closed headphone that's also comfortable.
 
V-Moda M80 isn't even a real over-the-ear headphone. Anything that rests on my ears or touches on them constantly is out of the question, because of the pain it's bound to give me.

We have very similar tastes and I found myself in the same position as yourself. This suggestion is left-field I know but it works for me. I bought a pair of Sennheiser PX100II's and fitted them with pads from the HD414. Yes they're on-ear & yes they're open but as a portable solution with a darker tone I have yet to hear better. They weigh next to nothing, apply very little pressure to the ear & cost £40. They're signature is on the dark side of neutral & they're the closest I've heard to the 650 yet. So much so that my V-Moda M100's are now up for sale. Don't rule them out completely.

solrighal said:

  We have very similar tastes and I found myself in the same position as yourself. This suggestion is left-field I know but it works for me. I bought a pair of Sennheiser PX100II's and fitted them with pads from the HD414. Yes they're on-ear & yes they're open but as a portable solution with a darker tone I have yet to hear better. They weigh next to nothing, apply very little pressure to the ear & cost £40. They're signature is on the dark side of neutral & they're the closest I've heard to the 650 yet. So much so that my V-Moda M100's are now up for sale. Don't rule them out completely.

Click to expand...

Glad to hear my situations sounds familiar. Far from ruling out PX100 II... I have actually been reading about them obsessively. Some people say it cures cancer. I couldn't believe the praise I heard from some head-fiers. On its amazon picture, it's nothing to look at, of course. But I believe them. I'll order one tomorrow, but I'm not in the states and it should be a while before I get it.
 
Now... Is it really that good?
 
There's a head-fier who feels that with PX100 II the famous diminishing returns are on. So much so that he has sold dozens of high-end phones and uses PX 100 II as his main headphone. Would you care to talk about them a little more?

Well, they're not the be all & end all but they are very, very good. I detest the current trend for bright sound signatures. The PX100II with stock pads has pretty strong bass. A bit too much for me if truth be told. That's why I bought the HD414 pads (Ebay is your friend). And it worked! The bass is now far more in line with the rest of the frequency range and they really are like miniature HD 650's. There's also no real need for additional amplification which, in a portable, I consider a good thing. They do respond well to my Fiio E18 though when I can be bothered taking it with me, delivering a bit more impact throughout the frequency range. It's far from a night & day improvement though and I usually leave the amp at home. At the price I think they're unbeatable. My V-Moda M100's were my first choice for portable cans but they have many weaknesses. Namely their build is nothing to write home about, they're very hot and, even with the XL pads, far too oppressive on the ear.
 
There are some headphones that are very comfortable but also carry a lot of mass which can become annoying when I'm mobile. They also cost £300 as opposed to £40 for the PX100II's. The Senn's weigh next to nothing and pretty much disappear on the head. Also, they fold up for packing away and despite they're apparent fragility they're actually very well assembled. If they have a weakness it's probably modern consumer pop music. I know, I don't listen to it either. They're not good at it though. Throw some Blue Nile or Patricia Barber at them and they don't even flinch.

The HD650's are too unique to really have a portable and closed equivalent. I feel as if the aforementioned Denon D2000 is probably the best reccomendation in this thread, in addition to a solid portable amp. I wouldn't go with V-MODA offerings if you prefer the sound of the HD650's, they are relatively bright and don't feature the clarity either, but that is entirely subjective. 

sennheiser momentum may be worth a try. Classic sennheiser sound and really block out ambient noise. may not cut it on your comfort requirement depending on your ears...

Thank you, Zorrofox! 
 
I'll get PX100II one of these days and will see how I like it. Watching as many YouTube concerts as I do, having a forgiving headphone like the PX100 II is a must. And it's cheap enough to throw around from laptop to laptop and not worry about an amp. Thanks for your help.