Well, I had a very similar situation...what I did was introduced myself, explained that I had a HT, told them I would keep it down, but rather than wait for a complaint..asked if they could tell me when/if there was a problem. I tried two set-ups, the second being with the rear speakers on the shared wall...and I played the system at fairly loud volume..late at night.
I kept asking, until the lady(40's) told me she'd tell me if there was a problem, but that since I moved the rears to the shared wall, they didn't really hear much.
I also put some drummer's padding(A pro eggshell type of deal) between the speakers and the wall.
The Sub is gonna be an issue, unless you use it sensibly...but if you ask somebody who knows accoustics fairly well, they will help you with materials and angles to minimize noise.
I was VERY lucky, because one of my friends is an engineer, and has worked with MANY fields of sound, so my neighbors never did complain..and the setup looked and sounded great.
Wall C worked for me, but I still maintain the best way to configure is to communicate.
Being courteous goes a LONG way..I am sure they heard the surround effects somewhat, but I feel that my efforts to try and make sure they did not experience any discomfort went a LONG way to maintain a healthy neighbor relationship.