Tonally, I have found the HD800 somewhat underwhelming in stock form. I thought my stock SR-Lambda, an almost 35 year old headphone, to have superior tonality. I love many things that the stock HD800 did, but it didn't ultimately satisfy.
Fast forward to the present, I have put Anax 2.0 (Rabid Dawg 1) and some simple and subtle parametric EQ and have fundamentally "fixed" this headphone. All my quips and reservations are long gone. The stock Lambda has nothing over the Senn at this point in time.
No more excessive brightness. No more thin and nasal tonality. All the technicalities and strengths of the stock HD800. The EQ shift was only about up and down 3dB. I'm sure EQ would really help the stock form also, but the mod makes the EQ simpler and less dramatic.
I am a newcomer to parametric EQ and realize that many do not want to have to learn the ropes, so I will post my exact EQ settings from Electri-Q (using Foobar2000):
Peak filter I - Freq: 4137.4 Gain: 5.8 BW: 1
Peak filter II - Freq: 5696.1 Gain: -6.1 BW: 1.1
Peak filter III - Freq: 20000 Gain: 3 BW: 3
Gain filter - Freq: any Gain: -3.3 BW: any
*The gain filter is to prevent clipping. Let your amp make up the difference by cranking up that volume knob.
Try it out if you can. You can even AB from no EQ (remember to volume match) to this EQ at the click of a button. It changed my HD800 from a nice but flawed headphone, to my favorite headphone. It probably would even help out the stock HD800, although not as dramatically, as I tweaked it with Anax 2.0 in mind. Also feel free to tweak and improve if you want. I aimed for neutrality and openness here, but you may want to bump the bass also.