You'll hear a difference between THX and non-THX speakers to the same degree that you'd hear a difference between, say, B&W and Dunlavy, Snell and Triad, and so forth.
The THX characteristics solve alot of potential issues which may or may not exist with non-THX speakers. That being said, you're far better off buying lesser speakers and putting the difference in proper treatment of your room.
You can put some Radio Shack Optimus speakers in an acoustically correct setting have some nice sound. Conversely, you can put a bunch of Meridian DSP8000 in a non-treated area and have horrible sound.
Of all your reproduction equipment (not *that* equipment), the room will have a more significant impact on the sound than any other component in the room.
It is absolute hokey to spend big dollars on cables, hospital grade outlets, better speakers, seismic sinks, and far eastern hockey pucks, until you address the room acoustics issues.
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