he AH-D2000 is somewhat thin and analytical compared with its more expensive siblings. Voices are a bit dryer sounding, not having quite the body and intimacy, and they sound a bit more distant than the other two. I also hear a bit of grain in the sound. Compared with the AKG K550 (a headphone of similar price and type) I find the D2000 to be more accurately resolving in the treble, but the K550 has a tighter bass and a more natural sounding midrange for vocals. The bass on the K550 may be a bit stronger than neutral making it a warmer headphone overall, where the D2000, while still a slightly warm headphone, has a more neutral balance in general.
The AH-D5000 clearly steps it up a notch in the sound quality department. Female vocals regain their organic warmth and juicy intimacy. The bass is slightly elevated and a bit loose, though now a bit tighter than the earlier D5000 model. The treble range is clear and clean, with a slightly overemphasized sparkle. This is a very good sounding headphone, but when compared directly to the Sennheiser HD 800 you can hear they're not as transparent, and the colorations of a closed back design prevent the airy depth achieved by the HD 800.
Moving up to the AH-D7000 is not nearly the leap in sound quality I found moving from the D2000 to D5000, but the D7000 is audibly cleaner, more resolving and refined. The tonal balance is slightly less warm than the D5000 and now very close to what I would consider neutral, though still a tad warm. Moving backward to the D5000 after becoming accustomed to the D7000, there's clearly a sense that the D5000 is a bit muffled sounding in comparison. The bass is tighter than the D5000, but remains slightly loose sounding. The mids are truly lovely and lush, but in comparison with the almost eery realness of the HD 800, the closed back coloration again becomes obvious. Highs have a touch of sparkle, but are a bit more refined and natural than the D5000.