I downloaded the firmware upgrade for my newly acquired 990.
Guys, do you think it will pe possible installing it may lead to a non working unit?
In that case I'd rather refrain from it, as returning it to the USA is almost out of the question...
Now, I once had an Arcam AVR300 which was a notorious difficult upgrader, but whatever happened it always reverted to the in-unit software stored in ROM, after a few resets that was.
Daresay that'd happen for the 990 as well?

@kgt
I did play around a lot with the sub settings on my (now to be replaced) Marantz SR9600.
As often advised, I've set the crossover to 60Hz and my center and rears to small, although these went down to 45 Hz.
But I didn't like the sound (bit mushy and thin) so I stuck with 80 Hz, all bass to the speakers PLUS to the sub.
This sounds best, and I guess it'll be even better because I believe I can set the 990 to different level for the sub in stereo mode.

It is possible for a hiccup at a certain stage of the process to leave the unit with no firmware loaded, but you can retry the update process after that and finish loading the new firmware.

You are exactly correct that you can adjust the sub gain for stereo mode - it's the setting called "2 channel sub offset."

Gonk:

How do you access this screen to adjust the sub trim for 2 channel analog ?

Go into the 990's setup menu, select "Surround Config" (about half-way down the screen), and go to the option "2CH Sub Offset" near the bottom. You can adjust it so that the sub runs up to 8dB lower in two-channel modes than it does in surround modes.

Gonk:

You are a wealth of information. Thanks again.

PS. Just out of curiosity, how did you come up with the name "gonk", gonk ?

"Gonk" was a droid in Star Wars, and my PC in college (a big heavy tower with more steel in it's chassis than four or five modern PC's) bore an all-too-close resemblance to that droid - as a result, we ended up naming it "gonk" at some point. When I needed a user name for an online forum account (ShackNews, although back then it was still ShugaShack), I borrowed my old (by then long gone) PC.

Great story..

I read a lot of rewiews from guys on the update, is there a list of all the updates somewhere?

yes click on the Model 990 link. and the top two threads are about the updates.