Well, you've got three (unspecified) fans plus (unspecified) HDDs in your current (unspecified) case. With the Antec 900, you'd have three 120mm fans, your hard drives, and a 200mm fan in the top. Unless you're currently using 80mm fans, I'd imagine that the two would sound very similar (at night), especially since you're main gripe seems to be with HDD noise, not fans. Notice that the front of the 900 is basically a mesh in order to let air pass - this isn't going to muffle the HDDs very well, and I didn't see any special neoprene mounts or anything to reduce vibrations, either.
So, from your post, it sounds like your 3 fans are quiet enough, but you need to either get some quieter HDDs or find a way to dampen them. There are several ways to go about silencing a HDD, including using plastic screws, rubber grommets (even a strip of electrical tape can help if grommets are too large/thick), or a special HDD enclosure.
Some cases (I apologize, but I don't know exactly which ones atm) use grommets in their HDD racks,
similar to this bay device. In the second picture you can see the inner rails where the HDDs are screwed in. Notice that there are large circular cutouts instead of small screw holes. There are rubber grommets that fit into these cutouts, and the screws fit through the grommets, so that when installed, the HDD/screws are not in direct contact with any metal framework. This particular Lian Li model does not fit every case. It may be worth trying to get a case that uses a similar type of HDD mounting method.