Initially bought the 300XS for my LCD-X. Then got curious with the HD800. The Sennheiser is like a "speaker" put near on to your ears. I did hear the hum but it has not really been a big issue for me.
After sometimes, I got bored with headphones and tried to find a matching speaker for the amp. The 300XS has a considerably low watt. Must admit that the choices out there are rather limited. My preferred option was to go for high efficiency speakers.
After sometimes I finally managed to find one that still stay within my budget. I bought Zu - Omen mkII bookshelf.
It's been almost 2 years now and I'm still very much in love with my Leben. I listen a lot to Classicals and Symphonies. The Leben is very neutral and it's definitely one of the amps worth noting.
I went on the same trip. I had a the original LCD-2s when they first came out and wanted a nice amplifier to pair them with. I started with vintage audio, receivers like the Luxman R-1120a and the Pioneer SX1250, both drove the LCD's great. Then I went to Leben but not the 300XS, but rather the 600 because I wanted something that could power speakers. Seems like a shame to buy a stereo integrated amplifier as beautiful and sweet sounding as the Leben and only use them for headphones. If anything, the Leben is a great 'gateway' to more hifi if not only an outstanding headphone amp. I went with Devore speakers, initially the Nines but upgraded to the O96. If you're in the market for high sensitivity speakers for the Leben I can't recommend the Devore Orangutan line enough, given you have the space for them. The Zu line also pairs well.
Be warned though, once you start with Leben you might get curious about that other brand called Shindo...