Hmmm, why isn't this in the manual?
FWIW-I have run a y-connector to create 6.1 in my setup for 4 years now with no ill effect.YMMV.
I understand that Outlaw might want to have someone produce a printing ready version of the manual (e.g. to get the graphics really nice). However, I can't answer why Outlaw doesn't have a copy of the manual that could be updated over time and at least posted to the website. To me this would be something, that an on-line company should do. That way as things change or errors are found they can be corrected and made available to everyone.
A couple of follow-up questions.
1) Do you use the Y connector before the 7.1 input or on the 990's left and right rear surround output?
2) Do you use a Y connector that is designed to do a mono summing (I have seen some Y connectors that are designed for the old stereo vcr to mono tv type connections)? By contrast most of the Y connectors that I have seen are, if they have any explanation at all, say they are designed to split a signal.
3) Did you do anything special in the setup? For example, reducing the relative output from each source?