I just discovered Mitch's convolution filters at Accurate Sound and I think they are great (mentioned a bit earlier in the thread). So great I bought one for each of my headphones (Abyss TC, Susvara, LCD-5, LCD-4). He is a former sound engineer and uses a measurement intensive process (and listening tests) to create convolution filters so each headphone is as flat as possible (up to around 6k because that's the limit without measurements specific to your head, I believe).
It was eye opening for me because I thought that while speakers are generally flat, that headphones aren't supposed to be flat - they need a preference curve.
I was wrong.
The LCD-5 one, I think, is the most popular because everyone EQs that headphone.
The Abyss one is my favorite, though, because the Abyss is my favorite headphone.
It does 3 things that I really like:
1) Cleans up vocals and places them a touch back in the mix. They sound clearer and in the right spot.
2) Fixes some of the wonkiness in the mids (e.g. there aren't any shrill notes that cause me to cringe now, it's all completely smooth and right sounding).
3) Improves the spatial characteristics of the headphones. It's surprising because this is where the headphone is already so good. But, this makes them even better. I think because everything is sitting where it should in the mix, imaging is better, and its inherent spatial qualities are more pronounced.
I tried it on SS: it's awesome. I tried it on tubes: it's awesome.
tldr: It's awesome. I highly recommend.