I'm happy with my Green Glue-treated HT. It stops almost all sound at moderate levels, with a little more of the bass getting through, as it is harder to stop, as mentioned above.
At loud, near-concert levels, there is some significant sound coming out - nothing would stop that in my budget - but it still sounds like the system is playing at a much lower volume than it really is.
I picture Green Glue as a really good set of earplugs. I should really do a test with an SPL meter to see what the dampening effect is. I'm thinking (:pulls number out of air:) at least 30 - 40 dB.
