The sound signature is, unsurprisingly, incredibly similar. The mids seem less recessed on the X00 Ebony, but this appears to be because the treble and bass have both been wound back slightly when compared directly to the 900. With a touch of EQ on either of them, you could get them sounding incredibly close to one another, whichever way you prefer it. Again, not that I'm advocating for EQ - if you're dropping more than $500+ on a pair of headphones you should probably just choose ones you actually like the sound of to begin with. The 900 outstrips the X00 in technicalities, noticeably but marginally (after all, it's always a case of diminishing cost-value returns creeping this high in price). The most obvious difference was the size of the soundstage - I'd have kept the X00 Ebony if they were wider. Apparently the Mahogany and Purple Hearts are even narrower than the Ebony.