Like Huxleigh says, they are two fairly different headphones. Materials and build quality alone justify the TH-X00 price... there is no plastic used in it except for the driver mount and two sleeve sheets inside the headband...all lightweight duraluminum and magnesium frame alloy that is lighter than plastic along with the exotic tonewoods. The HD6XX is all plastic other than internal headband. HD6XX is sturdier and more robust than the TH-X00 though, really need to treat the TH-X00 with care all the time, while magnesium alloys are fairly strong and very light, a little too much force and it will just shear and shatter apart - will not bend. Don't want shock forces on the magnesium parts.
For my modded TH-X00PH, bass is the biggest and most noticeable difference from an HD6XX, mids are kind of similar (preference is my modded TH-X00PH - more forward mids with a more enticing sound), highs are a little different and slightly more favorable in some ways on the HD6XX (smoother, more detailed, less fatiguing). The HD6XX is just not as clear or dynamic sounding though. And no getting around that biodyna bass in the TH-X00, which is the main reason you get them - for that massive sub-bass with impact yet still very detailed and finessed . While the HD6XX are competent in bass, they are no where near the level of a TH-X00PH. The reason you get the HD6XX is for their lovely and smooth mids and highs.