The are some sub-categories where the LCD-XC is competitive, mainly mid/upper bass layering and instrument placement/separation, but with regards to quantity, slam, decays, bloom, reverb, TH-900 pretty much walks it. It just goes lower. Possibly personal preference, but I've never quite gelled with the way planars cut the delays short or the way they dither note decays. The LCD-XC also gives the impression of having more of the bass (actually, this applies to all it's frequencies) in front of you, while the TH900 is more enveloping, more "from the gut" in that regard.
It's the mids and highs where the XC comes more alive, to my ears. Partially thanks to their speed, which is impressive. More on that later.
Disclaimer: I'm a basshead, so there's probably some bias here. For example, what some listeners categorize as "controlled", I might simply call "lacking". Neutrality lovers might possibly see those as XC advantages, though it's still far from being a neutral headphone. Like I said, it's apples vs oranges.