You are probably better off posting in the 990 channel under Legacy Products:

http://saloon.outlawaudio.com/ubbthreads/ubbthreads.php?ubb=postlist&Board=35&page=1

But it sounds like you have the right idea. The bass management matrix for the 990 seems to pretty clearly say that your sub will be used in all modes when setting the front speakers to 'Small'.

You can see that here:

https://www.outlawaudio.com/support/990_analog_bm.html

https://www.outlawaudio.com/support/990_digital_bm.html

Good luck!