I agree with braidkid - I'd use the two unbalanced sub outputs and not worry about it, especially since your SVS's most likely don't have balanced inputs and you'd have to add an adapter (which gets into a can of worms that I'd much rather leave closed). You've got so many adjustment points for the subs (gain control on the amps as well as trim adjustment at the 990) that any level difference can be sorted out without difficulty.