I always wonder if I should get a hd800(or the S now) and EQ it. I already EQ the hd650 and my favorite sound is somewhere in between those 2 signatures. I find both comfy and light enough so that's like 80% of what matters to me. but then if it's to still end up with the signature I enjoy with EQ, is the price difference something I can justify? I have a hard time saying yes. I've been leaning toward one direction then the other for the better of the last 2 years. the main difference is that I could use the hd650 without EQ. but I can't stand the hd800 as is. but no doubt the hd800 is an amazing headphone when it comes to signal fidelity out of a dynamic driver, it's really there at the top.
The HD 800 is definitely a better headphone but honestly sound quite different even when modded/EQ'd to sound more like the HD 650. A cheaper option may be to grab a pair of Nighthawks or something else with a better driver and distortion specs than the HD 650 and EQ them.