The Little Dot 1+ is made for low impedance headphones such as the X2. It is a tube/solid state hybrid amp. It drives the X2s no problem. I also have a mk2 and it does sound ok, but not great. This is because it does not put out the current of the 1+, being a tube amp. My package says the X2s take a max of 500 MWs. The Little Dot will puts out 800MWs. Though to me, it feels like it put out a little more than this.
I think the Little Dot sounds amazing. It really gives definition and extension to the treble and bass. I also thought the detail, timbre and texture overall was improved. Music was more dynamic and had punch. This was felt even at lower volume levels. I feel this is due to the higher current and the lush timbre from the tubes in the Little Dot. I have four really nice speaker amps with headphone outs and a very good preamp that was designed for headphones. The little dot blows them all away to my ears. The purchase of this amp was what really made me enjoy headphones. My other sources sounded tame compared to the Little Dot. Oddly enough, my various speaker amps, did not transfer their quality to my headphones. For around $130 with shipping included on Amazon, its a great deal. I would recommend putting in a Muse 02 opamp from Mouser electronics or Digikey. I have tried several opamps including a discreet one. The Muse 02 easily beats them all to my ears and in my opinion, the modest investment takes the 1+ to a whole new level.
The amp is also DEAD quiet. You can get noisy tubes though. Some have a slight hum till you burn them in for 8 - 48 hours. I actually like the 6j1s it comes with after a little burn in. Tube preference is a very personal thing. The X2s seem very forgiving regarding their source.