The best place for a sub is usually in a corner, particularly a corner comprising of two outside walls (these are generally thicker and more rigid than a single brick wall). You can get quite a lift from a sub placed in a corner - usually in the region of 3-6dB. This means that the sub can sound at least twice as loud at the same volume, so it doesn't have to be pushed as hard. In some rooms this can boom a little though, and can give its position away, but it's worth trying. I can usually hear where the sub is (not the subs fault, but I can usually tell), so I always try and place mine as central as possible, and always at the front. I will usually recommend placing the sub anywhere along the front wall with the front speakers. I usually go further and say that I'd recommend putting it between the front left and right speaker somewhere, but as many people have to compromise with the placement of speakers in one way or another, I'll say anywhere along the front wall, corner to corner.