Personally, I prefer the sub to be more in the vicinity of the front speakers. Often just to the inside of the left or right speaker. So, I'd put it to the inside of your left speaker, facing forward. I know it's common to believe sub placement doesn't matter, but that's just not true in my experience. As Dave said, you never want it in the middle of a wall. Typically 1/4-1/3 of the way from the closer wall is a good place to try. Also, if it's near the front speakers, then it's sound level disperses the same as the front speakers. For some reason, people tend to believe the sound from subs doesn't diminish with distance, but it does, just like any audio. Unusual placement tends to result in much higher levels in one sitting position and much lower in others. Placing it near the front speakers eliminates that, since the speakers are all the same approximate distance.
FWIW, on top of an equipment rack doesn't sound like a good idea, since the rack isn't a very solid foundation. The whole thing is going to vibrate like crazy, and your equipment with it. Imagine a disc player trying to track while it's being vibrated directly by a subwoofer. Do you really want to subject your TV to that? I wouldn't do it.
FWIW, on top of an equipment rack doesn't sound like a good idea, since the rack isn't a very solid foundation. The whole thing is going to vibrate like crazy, and your equipment with it. Imagine a disc player trying to track while it's being vibrated directly by a subwoofer. Do you really want to subject your TV to that? I wouldn't do it.
