Now I'm completely baffled.
I just completed the firware update - installation successful and new version confirmed.

When I set all speakers on the 990 to LARGE and SUB to LFE, I can control all levels independently including the SUB by pressing the TEST button on the remote.
Is this all happening on the analog domain? or is the 990 doing an A-D-A conversion to the analog SACD signal?
I thought setting all speakers to LARGE kept 7.1 in the analog domain.

Also, is there a definitive list of changes that the update implements? if not, why?

Well, I thought that channel trim was a digital process, but that doesn't mean that it actually is the case - after all, the main volume control is done in the analog domain. You might ask Outlaw, either by phone or online ...

As for the list of changes, I believe that the quote below (taken from the announcement for the new firmware) is the summary of changes for this firmware update.
The support page still lists the changes implemented in 3.10/3.11, so it would be worthwhile to update it - possibly with the specific changes spelled out.

I've seen this confusion in many forums. "Digitally controlled" level controls doesn't mean they operate in the digital domain. The signal is analog, only the level control chip is being given a number on a control bus instructing it how much to attenuate.

Big names like Krell use these type of volume controls instead of a motorized "classical" potentiometer in some of their stereo integrated amps and not only.

Definitely the fact that you can adjust the levels independently doesn't imply an ADA chain in the signal path.

Gonk and Alexandru, thanks for your replies.

I will confirm with Outlaw and post if they say anything different.