ie. a sub must go down so low and so loud for so long a time at a certain level of distortion....
An amp must be able to generate a certain wattage into a certain load and still be stable etc.
There are electronics that exceed THX standards, but elect not to pay for the licensing of the sticker.
For the most part, THX is noting more than a cash producing machine for George Lucas.
Now THX speakers are another story as they have to conform to a certain dispersion pattern (with waves hitting neither the floor, nor the ceiling). While GREAT for movies, it is detrimental (for the most part) for music. That is one reason that the really serious (and well-heeled) audiophile will have TWO systems in his house. A Home Theater and a dedicated stereo.
With the advent of SACD (Super Audio Compact Disc) and/or DVD Audio with high resolution 5.1 recordings (along with future speakers that will have built-in microphones and high speed digital "equalizers" to "listen" to itself and adjust to room conditions) this will change.
BTW...the room that equipment is in determines 70% of how a system sounds. I can take a properly adjusted $2000 system in a 'good' room and blow away a $40,000 setup in a two story atrium family room.