Wow, there seems to be a lot of confusion about this. Whether to calibrate to 75 or 85db depends on which calibration tones you use and what your listening preferences are.

In the case of Avia, the tones are recorded at 85db on the reference scale. That means to get playback at reference level for movies, you should calibrate your speakers to 85db using these tones.

It's entirely possible that the THX optimode and/or Video Essentials test-tones are recorded at 75db on the reference scale, but I don't know that for sure since I don't really use those. But if that's true, then calibrating with VE at 75db would give you exactly the same results as calibrating to 85db with Avia. That's the whole point of "reference-level", it's a reproducible standard. So the best advice if you want to calibrate to reference level is to follow the instruction that accompany the calibration disc you use. The AVIA disc has very good documentation available from the menus for each test.

Now for many people, particularly in smaller home theaters, full reference level will seem very loud with today's action movies, probably too loud. That's fine, you can always turn the master volume down when watching movies. Theoretically, you are sacrificing some dynamic range by not listening at reference level, but in reality few if any movies use that full 105db of dynamic range. Most of todays action-type movies are mixed loud; the sound level is at the upper end of the dynamic range for most of the movie. Dialog-driven movies aren't going to be as loud but they still probably don't use the full dynamic range.

As for the THX guy who said to use 85db for music and 75db for movies, I think you must have spoken to a clueless marketing guy who didn't know what he was talking about. CD audio does not have a reference standard for volume, that's why volume can (and often does) vary from disc to disc. Maybe DVD-A and SACD have a reference standard, but I don't have either so I couldn't say. But for CD audio there's no such thing.