Stuart and J, looks like the bass sync is not what we probably expected. The 4520 uses context-sensitive items in the setup menu. Bass sync is a sub-menu of "Surround Parameters" and this menu option only appears when the audio source is multi-channel. When using REW, I am using a stereo setting on the AUX1 input, and cannot get the Surround Parameters menu to even appear. I even tried changing AUX1 to a PLII setting, but still no luck in getting the bass sync option to display. So, I can't measure it. Besides, if bass sync changes depending on whether the source is stereo or multi-channel, that wouldn't be desirable, would it? I'm not going to fool around with this setting any more, unless someone has a good idea on how to measure its effect.
Now Audyssey Low Frequency Containment is a different story. LFC is turned On or Off in the Audio/Audyssey setup menu screen, and when it is on, the aggressiveness of the LFC can be set from 1 to 7. Check this out:
Obviously, there is some serious low frequency containment going on here. For anyone having trouble with the sub bottoming out when playing Tron:Legacy, this is your answer! The alteration in the frequency response between 180Hz and 300Hz is somewhat concerning. If the objective is to keep bass within a reasonable level, I would experiment with Audyssey DEQ RLO, or Dynamic Volume, before I would engage LFC, IMHO.