In the pro audio world, flat/neutral sound is the holy grail for production, mixing and mastering (especially the latter). Hence SW was born. It was initially designed to calibrate studio monitors within your own room (each room having individual sound characteristics / reflections etc). The SW then took multiple versions of the same headphone and took specific measurements of their frequency curves, giving them a very good average reading for each headphone. This allowed them to produce a curve that, when applied to the stock HD800 sound, would flatten it so the desired reference level.
The HD800 is particular is
especially responsive to EQ, it scales like no other headphone. Quite remarkable. It's known for being bass light with a 6kHz treble peak, when you run it through SWR3 that character is completely changed to one that is very, very accurate. The bass it can produce is mind blowing! You still retain the massive soundstage and accurate tonality.
The stock HD800 is not an accurate sounding headphone. You may be used to it, but that's as far as it goes. When you apply SWR3, it will at first sound a but dull in comparison as you've normalised those high frequencies. Stick with it for a few hours (days preferably) and then pop back to stock to see how bright it really is.
For the record, I have a high end studio monitor setup in an acoustically treated room. The balanced SWR3 V281 HD800 combo is very close. Crazy good.
Some tips: Under the Advanced tab, make sure you're in Linear phase more. More accurate. Also make sure 'Avoid Clipping' is enabled. You'll have to turn your volume up to compensate, but it stops the audio from being clipped (bad). Think of your stock HD800 frequency as a full glass of water. When you start to remove/add additional frequency via EQ, you're essentially tipping out of adding more water. If you add so much as to make it overflow (clip) you lose data/quality.
If you're using Mac, the Audirvana music app is the best choice, and you can add plugins via the settings. If you're on Sierra OSX, you'll need to disable Direct Mode in Audirvana for the best sound quality (bug in that version of OSX). You can also use Audio Hijack, more complicated, but let's you run SWR3 across your whole system, or specific apps (such as Safari, Spotify, Tidal, VLC, PLEX etc).
The sound 'flows' from left to right. You place stuff in specific orders and can toggle inputs on/off easily. The Ghz plugins you see between SWR3 and the DAC output are also fantastic, they let you emulate a speaker sound via headphones. Another great tweak to make the sound that much better.
Foobar lets you use plugins in Windows, if I'm not mistaken.
If you have the V281 nd the HD800, get yourself a nice balanced cable too. Makes a big difference!
The official thread is here:
Sonarworks Headphone Calibration software