Good morning Roger, after a good night sleep, I think I may have found the answer.
My theory is it's related to 2 aspects of the testing:
1. My speakers' crossover points
: main at 40 Hz, center at 60 Hz, and surround at 80 Hz. The speakers that come out higher with THX are the ones with the high crossover points.
When Theta's internal noise generator is invoked - two aspects are different from THX signal:
***a. Theta's signal is narrow band
***b. I have to check again or someone from Theta could confirm, the signal say for left surround is sent to the left surround speaker only
, and not through the subwoofer. Contrast this with signal from AIX disc, that being an external signal from a blu-ray "movie," is sent through both surround and subwoofer
. Hence, at same volume on Casablanca, AIX is louder.
I believe that if b above is true, it is the main reason for the discrepancy. And if b is not true, then a is the reason.
Why I believe I am on the right track, fingers crossed:
1. The subwoofer level test match closely between Theta and AIX.
2. The speakers with low crossover point, my main speakers, match closely between Theta and AIX.
3. The speakers that do not match are the ones with high crossover points, my center (60 Hz), and surround (80 Hz).