what the THX ?

It stands for Thomas Holman's Xperiment.

This guy made a great amp & preamp in late 70's. Later, he went to work for George Lucas's Skywalker Ranch & developed the THX standards.

The initials stand for Tomlinson Holman's eXperiment, so dubbed by George Lucas.

The name and number associated with this is an interesting thread that runs throughout Lucas career. Way back in 1967, Lucas did a student film titled THX 1138 and then remade it and released it commercially in 1970.

I saw the film the first month of it’s release and I can still remember how stunned I was at its creativity (for the time). A very young Robert Duvall played THX 1138.

For whatever reason, George Lucas loves these initials and numerals, using THX for two movies and his pet surround sound project. The numerals 1138 occur again as the number of the cell block in Star Wars, and the license plate of Paul Le Mat's deuce coupe in American Graffiti.

Maybe a better question would be why does George Lucas love these letters and numerals?

As Beavis notes, Tom Holman designed a pretty nice preamp back in the 1970's. It was called the APT Holman preamp, and I had one for about a year and liked it very much. For it's time, it was a very good unit.