Couple more options.

With a pair of Y adapters such as this http://www.outlawaudio.com/products/cab_y_about.html you could split your un-balanced output to your amp and run a full range signal to your sub. This would accomplish the same thing that I have by using the speaker level input. I plan to use the balanced connection between my 990 and amp so this wouldn’t work for me. If I could use the balanced connections for the amp and the un-balanced connections for the sub I might use that. That is a question that I would need an answer to before I tried it.

Another option would require the un-balanced connection from the 990 to the subs via the Sat/Sub input on your sub and then another cable from the Sat Output on the sub to your amp. Depending on where your sub is, this may not be very convenient as far as running cable to and from the sub.