Thanks, but they've already come out with successors to those headphones.
Doesn't matter. This isn't like computer hardware - newer is not always better (fortunately for us). The T70 (the replacement for the DT770) is very bass light, hard to get a good seal on (and when you can't, there's zero response below ~200hz), and incredibly harsh with lots of electronic music. It also costs three-four times as much because Beyerdynamic is leading the charge (teehee) towards getting you to spend more for less. Based on the T70, I wouldn't even bother with the T5 (that, and when Tyll calls something "an abomination of harshness" I tend to pay attention). It's built very well though, and comes with a nice case (that you can buy for $30 direct from Beyer).
DT770 is a good suggestion for what you want. The Denon models provide no isolation, and are too fragile for true portable use; especially the D5000. Really depends on how "portable" we're talking about. Ultrasone offers bassier headphones that are built like a brick. But they cost more, and if S-LOGIC doesn't work for your HRTF, they will sound very alien. Try the DT770 or the Pro900 (or one of the less expensive Ultrasones, like the DJ1Pro). Musical enjoyment does not have to cost thousands of dollars (as a great man once said: the point of diminishing returns is in the thousands, not the billions!). Especially for portable use.
Finally, just to throw it out there (since you did say portable, and I see this so-often overlooked for portable use): have you considered IEMs of some sort? The money you're looking to drop could translate into custom-fitted units (which I've never personally had made, but I understand their advantages for providing isolation and comfort), or very good universal-fit units (which cost about a tenth of what you're looking to spend) that will conveniently drive from any portable device you've likely got. The (substantially better) isolation abilities of your generic Ety/UE/etc coupled with the generally great bass response of many IEMs could absolutely get you where you want to go, in a more easily transported package, that doesn't require substantial additional amplification, and likely won't shout "hey, come mug me!" like a Pro900 or D5000 would. IME, in-ears are the way to go for mobile use, or any high-noise environment for that matter.
But I had read something like "more/better control [of the diaphragm, but I'm not sure] for the higher impedance headphones" and that day, came to the conclusion Beyerdynamic made their flagship a 600 ohms headphone for that reason.
Although I now think I shouldn't have embarked myself on a subject I'm so ignorant about, lol. Sorry for not having anything productive to add, and for skidding on the topic.
Beyerdynamic aside, impedance (when people stop saying "ohm rating" I will be truly happy) means nothing for actual performance. Ohm's Law dictates how voltage, current, and power will relate to one another based on resistance (measured in Ohms) - impedance is the complex AC resistance of a system (it varies wrt frequency).
In determining amplification requirements, impedance and sensitive must be considered. While you cannot arbitrarily have "lots of volts" or "lots of current" (something that people seem to prescribe - I'm not saying anyone has done that here, but it's usually something that comes up in such discussions ("oh get an amplifier that can provide a s*tlot of current to clean up the bass")), you can have a situation where a load demands more power at a given output level based on it's impedance (and Ohm's Law). This can result in the amplifier going into clipping (or blowing up). In other situations you can have a load that is too reactive or too resistive for an amplifier to drive in a linear manner.
An extreme (but not unrealistic, unfortunately) example from a Rod Elliot article creates a hypothetical 100W/8ohm (28V out) amplifier, and hooks it up to a speaker that has an impedance dip to 1 ohm. The power demand placed on the amplifier at that load is around 650W (after cable loss) - that's impossible for this amplifier, obviously. That's a problem. With headphones, this is usually less serious, because power demands are usually minuscule, and even with highly inefficient SET Class A designs, a few watts is not inaccessible. Additionally, headphones are generally not this poorly designed (they don't have a crossover to screw things up, and many of them measure fairly flat). Usually what you see instead of extreme impedance drops, are large humps where resonance or other nasties are encountered - the Sennheiser HD 600 is a prime example of this. Where impedance comes into play there is how the amplifier's voltage drive abilities can hold up to a fairly large up-ward swing in impedance (which also happens to be at a lower frequency, which is where more power is demanded on average); if the amplifier cannot provide the voltage needed for the desired output power, it will limit. Additionally, the inter-relationship between the Zout on the amplifier and this impedance can (and does) change the FR of the system by attenuating things in a non-even manner. This is "coloration."
Usually this is not a huge problem though - most headphones need far less than 1 mW of continuous input power to reach substantial output levels. So as long as the amplifier can provide that 1 mW into a range of loads, it will work (and we don't have to do any deeper digging). For example, the Creek OBH-11 specifies 10 mW into 30 to 300 ohms; I have no idea if that's verified, but if we just assume it works, it would be able to satisfy the swings produced by something like the HD 600 as long as your (acoustic) output requirements don't exceed what 10 mW can provide. Given that the HD 600 is somewhere around 97 dB/mW, 10 mW should be quite sufficient (figure you will never listen at 97 dB nominally, because it's dreadful, so chop that figure down by 20 dB to .1 mW and you've got a massive amount of dynamic range left-over).
You can scale this example to anything you like, such as the T1. You just have to know the impedance response of the system and what the amplifier can do. This does not define or explain the universe or all differences between amplifiers, mind you.
Now, in terms of "higher impedance for more control" - Beyerdynamic specifically makes this claim (or I should say, I have heard this claim associated with the DT line) regarding their various impedance offerings. There's not really any other model out there that has multiple versions of the same driver or system at different nominal impedances, so it's hard to make generalizations beyond what we can witness with Beyerdynamic. Throwing series resistors in-line with something else, like an AT or Grado, will just attenuate the entire system. If the load is relatively reactive, the attenuation will be non-linear and you can fairly dramatically change the sound depending on how much resistance you put in (and remember, if you put too much resistance in, you'll cause the amplifier trouble - you're asking it for much more voltage to drive the same load (so don't be a yum-yum and stick a 10meg in there!); also figure the power dissipation and pick an appropriate resistor if you want to try this). The only validity I can source for the claim overall though, is that higher impedance headsets generally reject noise better and can interact with higher Zout sources with better success (e.g. there's a reason you don't plug IEMs into a big OTL).
In the case of Beyerdynamic specifically, I think the reasoning is more along the lines of the equipment they target those devices for. Their marketing literature promotes the 32 ohm units for mobile use, things like iPhones, that have a limited Vp-p and may choke if you ask them to drive a high output level from something with a relatively high Z (Because it wants more voltage than they can deliver - nevermind that some users report mobile devices lack the current drive for those cans!). The 600 ohm units are marked for home hi-fi systems, which usually derive their headphone output through a resistive network tapped onto a large power amp (that probably has a fairly low Zsource to start with, but it's driven up through the sky to protect the hp's). These devices can usually swing more than enough voltage though, so producing a headset that will play nicer with them is a logical conclusion.
If you have an hour or two to kill, PM me, and I'll dump some links on you.
