Almost all the common equipment available and frequently used in commercial cinemas is already on the THX certification list. The theater does not automatically get blessed with the "THX Theater" certificaiton unless they meet some other acoustic criteria during the construction of the auditorium. And of course there are some costs involved in the actual certification process.

So depending on the construction of the theater, since the same equipment (speakers, amps, preamps, etc, etc) is used, a non-THX installation and one that is blessed can sound identical. The only other variable is the construction of the auditorium itself with things like reverb time, isolation etc being specified in the THX spec. Also the screen is specified, along with it's brightness.