Six-Channel Fan Control: Scythe Kaze Master Pro
I wanted a fan controller that'd at least somewhat match the 900D's design. After mixed experiences with touch-enabled controllers, I decided to grab one with knobs: Scythe's KM04-BK. It isn't cheap, but who would put a discount fan controller into a high-end case, anyway?
Six channels are exactly what we needed to support four case fans and the two radiator coolers. Although you can mount normal 5.25" peripherals (optical drives, for example) in the Obsidian 900D without using screws, that's not the case for shorter accessories like Scythe's controller. Screws were necessary, but they aren't any sort of hassle, and they actually improve front-panel alignment.
A word of advice about the fan connectors: it's all too easy to pull the power and sensor cables off of their respective headers when they're routed inside this case or any other. Prevent that by tying the cables together and attaching the bundle to the fan controller enclosure, as shown below. Corsair includes cable ties, and this is a good use for one of them.