Hello Rich,
For the VP180, I'd keep it out about 10 inches or more. You can listen to male speaking voices (dialog), and if the male voice is at all "full" or "plummy", or too rich sounding, then you are getting some bass boost or emphasis from the adjacent wall. So move it out a bit until that fullness is reduced or eliminated.
The location of the subwoofer in the room should be where the deep bass is the smoothest and non-boomy from your listening position. As you may realize, the subwoofer bass is non- directional, so you won't "hear" the location of the subwoofer so long as you set the crossover to 80 Hz or lower. Although you may think bass is directional, the directional qualities are supplied by higher frequencies from the main speakers and center channel, not the subwoofer with a low crossover setting.
Putting the sub behind the screen is not a good idea, as it limits your choice of the where the subwoofer should go in the room for best performance.
Regards,
Alan
