This is my first dedicated HT, and I want to enjoy it without having to worry about it being loud enough to piss off the neighbors.

I have one shared wall. Would hanging thick blackout curtains on that wall help?

I have one shared wall. Would hanging thick blackout curtains on that wall help?
It might help make the acoustics inside the room a bit less reflective, but blackout curtains won't provide any meaningful soundproofing. That's a much more involved process, unfortunately.

It's really hard to give much feedback based solely on the information that it is a common wall. What can you tell us about the constructions details of the wall and ceiling in that room? Drywall construction or concrete? Any insulation between the units? Shared HVAC runs? Can you hear any noise coming from your neighbors?

foam padding or the like, underneath the speakers.
been doing it for years in my apartment. no complaints. yet.
but i dont crank it really loud, either.

you need to read the library of articles here. Then you will have some knowledge of the difficult task you face.

www.soundproofingcompany.com

Just my uneducated opinion but based on old experience.

Your front, center and rear speakers mostly output higher frequencies which are most easy to block and are more directional in nature.

Lows from your sub, on the other hand, are much harder to insulate/contain and most likely to be heard (and felt) through a common wall.

This is why the shared wall and other shared but not seen building components like ceiling trusses or air distribution ducts which can serve as conduits for sound.

We always hated living in multi-family buildings with shared walls - both because we were limited in our enjoyment of "cranked" sound and often forced to endure the audio pleasures of our neighbors.

Our problem solved only when we bought our first free standing single family home.
Best of Luck

How much sound would be reduced by adding an additional layer of QuietRock 525 (http://www.quietsolution.com/html/quietrock.html) or similir on the shared wall? The other walls aren't shared and the ceiling is concrete.

ETA: After reading up, it looks like green glue and an additional layer of sheetrock may be the most cost-effective solution. Opinions?

If this is the best option, what should I expect to pay to have this done? The wall is 18' x 7'8".