I was playing around with my system yesterday and I ran a speaker level check using the internal tones from my yamaha amp, the center was up by 6db and the surrounds down 2db from the mains. Last time I calibrated the system I used the Ultimate DVD platinum so I popped that in and ran the test again, all speakers were level matched. I know that it is better to use a disc to set levels rather than the receivers internal tones so I left the setting as is and started playing around with the TPM DVD. I got into the THX optimizer and ran the test tones again, the results this time the center -2db and the rears at +2db from the mains (There is a note on this screen that says the tones are not intended for speaker level matching). I then popped in a third setup disc, one that I got for free with a DVD mag, this time the center was +1db and the surrounds +1db. Im thinking that this is being caused (mastering errors aside) by certain frequencies in the test signals being excited by room acoustics, as all the tones seem to be in a different freq range from each other. So it seems the only way to do a true calibration is to first get a flat frequency response from all speakers using equalization and then calibrate using a setup disc(s) - all three DVDs should then give the same results. Sounds like a pain in the arse! So much for speaker calibration being a five minute job. Harry.