You might consider a new or used Outlaw ICBM to help out, placed between the 990 and your amplification. The ICBM is only 6.1, however, one can set all speakers to large except for the rear surrounds. The 990 outputs for the rear surrounds are run directly to amplification. The remaining 5.1 of the 990 are sent through the ICBM set up for stereo subs. The sub frequencies for the rear surrounds have been directed by the 990 to the subwoofer output of the 990 and are carried to the ICBM in the .1 channel. One ends up with stereo sub reproduction except for dual mono sub for the rear surround only. Any LFE's have been included all along.

If one then feeds a 5.1 direct source, such as SACD or DVD-A, then it is able to pass through the 990 largely unaffected and have 'stereo sub' analog bass management applied after the 990.

I would like to see Outlaw to begin selling a 7.1 version of the ICBM.

Edit: ICBM does not have balanced audio connections. Maybe something to add to a "7.1 ICBM?"