Audigy 2 ZS, 2 + sub output?

[beatle]

I have a receiver set up that allows to switch between 2 sets of front speakers. I want to use this to do an A/B comparison with my Z680 satellites and a set of Cambridge Soundworks Ensemble IVs. I'd like to keep my sub on, as neither of these speakers produces low frequencies very well. When I select 2.0/2.1 speakers in the speaker control panel, it kills my sub. When I look at the bass redirection tab of the setup, it is greyed out. Alternatively I could just unplug my center and 2 rears, but I'd rather do it in software.