Don't have the X00's yet, but do have Denon AH-D2000 and am in on the MassDrop Purplehearts. Since I have the Purplehearts coming in the future and decided to get some harder to drive Fostex T50RP Mark III in the meantime, I just got my first headphone amp, like a few days ago. The Grace Design M9xx was one I looked into, but ultimately went with the Meier-Audio Corda JAZZ-ff because it was designed to power more difficult headphones, was interested in its Active Balanced ground driving setup and clean circuitry layout, I was just wanting an amp since my Yamaha DVD-S1800 has an excellent DAC section, and it was noticeably less expensive.
I tell you what, these AH-D2000's (which I've modded) sound excellent on the JAZZ-ff amp fed from the Yamaha DVD-S1800. First thing right off the bat is with the gain on low and volume knob all the way down it is already outputting sound at a decent volume with the AH-D2000's. Secondly, the phones have never sound this clear before, veil lifted and micro-details come out all over the place. Thirdly, the bass is just too much now, I can hear where the bass reproduction of the phones stumbles more easily now when driving them hard. The phones are not as tight and quick in bass as I thought they actually were before now, but man do they have some impact and more sub-bass! If the Purplehearts have 'less bass' but tighter response I will be extremely satisfied, the Denon AH-D2000's are kind of overbearing in bass when driven hard, great at low volume though.