Bass for 2channel

Currently I am not using either sub. for 2channel music and I would two incorporate one sub in the mix. How can I accomplish this?
I know stupid question but I can't figure it out.

Thanks in advance

The 990 has a 2.1 upsample mode which I use for two channel music to include my sub. Just connect the sub to the output of the 990 and set the xover in the 990 to about 60 hz. Select 2.1 upsample as the listening mode on the 990 and the sub should be there.

Thanks XM !