Again, I think the 6XX has a place but it boils down to genres of choice. While they excel at rock, acoustic, and classical, I think they fall short with anything else. They don't do well with Hip hop, soul, funk, RnB, blues, pop, metal, etc. I listen to much more of the latter and the 6XX are just not up to snuff enough compared to other sets that I own that are superior eg. the Elex.
I'm also not terribly fond of how the 6XX does with female vocals: Nina Simone, Etta James, Howlin Wolf, Big Mama Thornton, Dinah Washington, Son House, etc. the 6XX manages to suck the life out of these artists and remove the unique qualities of their voices. Same with certain instruments like Keb Mo playing the steel guitar or Booker T. on his Hammond B3 organ or Howlin Wolf on the harmonica.
With all that said, I still think the 6XX is very capable but that really boils down to what you listen to and your audio chain. As Jackula stated, these really don't take on their true character until you have them powered through some pretty expensive gear. Most of which the avaerage listener will not have. For the cost and means of use, there are better alternatives which is all I'm saying. I took a gamble on the Elex and they are my favorite pair. They're easy to drive, aren't amp picky and sound quite amazing being driven by even the most humble of setups. Throw in build quality, comfort, and their fun, dynamic and technical sound signature and you have a true sub $1k knockout.