My current phones, the HD800 are hot off the press (my second set of these) with Serial 314xx

First few hours noted that compared to my previous set with SN 245xx, this had a flatter response. It also seems to have a slightly reduced imaging / spaciousness - need to break it in. I applied for the freq. response and sure enough, this unit had less peak at 6kHz, a slightly higher 10k and overall, smoother response.

To the tweaks:

Tweak 1
This is a really simple, super quick one. I've measured that the further forward you place the HD800 on your head, the less peaky the 5-6kHz gets. Personally, I place the phones such that the back of my ears are just touching the pads.

Tweak 2
You need
1. Foobar
2. Graphic Equalizer Component (foo_dsp_xgeq)
3. Audiocheck.net downloads of the following Sine audio files: 1000Hz, 1250Hz, 1600Hz, 2000Hz, 2500Hz, 3150Hz, 4000Hz, 5000Hz, 6300Hz, 8000Hz, 10000Hz, 12500Hz, 16000Hz (and 20000 if you want to test your hearing).
To save you figuring it out, and me answering questions, I've attached the files.
https://dl.dropboxusercontent.com/u/90236454/Sinewaves.zip

Install the Graphic Eq Component. Load the sinewaves in your playlist.

Using your ears only, in a quiet environment, play each file whilst using the EQ to try and get them all similar level. Here is where we all differ due to our different ears, heads, hair, and of course headphone variations. Here are my settings:

2.5k -3dB
3.15k -3dB
4k -2dB
5k -4dB
6.3k -5dB
8k 0 (no change)
10 -2.5dB
12.5k +4dB
16k +4dB (My hearing needs more but I don't want to over boost)
20k +4dB (Can't hear it but I didn't want to boost it any more and mess too much using DSP for no real gain.

And for countering the rolloff at 20Hz, add up to 5dB there.

You may need to do this adjustment a few times over a few days as you'll need to give your ears & brain a break. And don't have the volume too high.

The issue most audiophiles, including me, is to keep the signal chain pure. However, after many years in this game, I've come to learn that modest EQ does wonders with little cost to sound quality. Certainly, it has greater benefit than any cable mod or amp.

You're probably aware that many HD800 owners opt for a tube amp to tame these phones. This works in two ways -
a) The high output impedance of the tube amp causes a boost around 100Hz plus a loss of damping (bass control). This boosts bass at one narrow band (not needed there) and causes bass to be looser sounding.
b) The higher distortion of the tube extends the decay of sound which for some music, is pleasant.

Both are at the expense of speed, transients and clarity.

Both are forms of added distortion.

So with a decent quality solid state amp and modest EQ to boost relative bass only where needed, tame the upper mid/ lower treble peaks, you come out with a neutral, agile, spacious, high resolution sound.

Mod 1
The first one is simple but requires careful soldering & tools:
What you need
a) A Neutrik or other high quality TRS male plug
b) Matching female plug (socket)
c) Quality solder (I use German WBT high silver content)
d) Heatshrink
d) Good stripping / cutting tools
e) Soldering iron, Magnifiying heatgun

Simple - cut the factory cable at the 1m (3ft) mark. You may prefer slightly longer. The shorter the better, but it needs to reach your desktop amp when seated. For longer reaches, you use the female plug on the longer length. Warning - the HD800 innards are a REAL PAIN to work with as they're very fine fabric/wire strands.
You get all the benefits of the short cable - one-third the resistance, one-third the inductance, one-third the capacitance. And when you need a greater reach (say in your TV room), you attach the custom made extension.
Or, you make a short cable from scratch if you're capable.
You will surpass any $400 aftermarket 3-metre cable for sonics at a small cost. Or get some professional to do one for you. (We do it in Oz)

Mod 2: This one I haven't actually done yet and is on the potential list and up for debate:
1. Remove the dust covers from inside the cups (reach behind the straight part of the D earpad)
2. With a good knife, cut the circular flat part of the dust cover.
3. Return the dust covers in to position.

The major benefits of this mod are:
a) Nothing between you and the drivers (which can be done without cutting the dust covers to experiment)
b) The covers still act to slightly diffuse / damp reflections off the steel mesh and trapezoidal piece
c) The baffle (adjacent to the driver) is uncovered and unaffected.

Pirates Ahoy 
Elmura Audio