We've done a number of designs with soffit mounted subs. There are issues, vibration, making a very rigid mount, but if you can over come those it can work well. In most cases the floor is simplier, and it's what most people do, but it doesn't necessarily mean it's best.
The 1/2 between floor and ceiling as Eric points out can sound very good. It cancels out the height mode which is typically the dominant axial mode. We do this regularly in our designs
The 1/2 between floor and ceiling as Eric points out can sound very good. It cancels out the height mode which is typically the dominant axial mode. We do this regularly in our designs