"In terms of a work-around, I asked if I could use a Y connector to join the output signals of the 990 into a single output. He said no that this could harm the 990."

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.
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990/Oppo BDP-83SE/Toshiba A3/Emotiva XPA-3/B&K ST-1430-II/Gallo Ref 3.1/Gallo Reference AV-Center/Gallo A'DIVAti(3)Rear/ MIT Term 2&4 Cable/Mitsubishi WD65C9//Monster HTS-1000.