im not andrew, but i'll have a go at answering. Thx is just the same as any other format (dolby, dolby digital etc etc) in that it is used by a film maker to record the sound. Therefor if you play back the film in the same format as it was recorded, it will sound just as the film maker intended. In the case of thx sound on your 875, there is a seperate spkr set up designed specifically for thx in the instructions where you need to have your front spkrs set at a level of 80 and your sub set to 80 also so that all "sounds" below 80 on the film go through the sub. If i have confused you, i apologise but it's all quite easy.