Since it's a stereo pre-amp, make sure what those three pairs of outputs are. I'd guess that with one pair being balanced and the others unbalanced that the balanced and at least one pair of unbalanced are "pre-amp outputs" (thus always active and downstream of the pre-amp's volume control). I'd be a little worried that one might be a record output, though, in which case it might not be active all the time and would not be affected by the volume control - thus making it a bad candidate for connecting to the sub. That's the only concern that I'd have about this arrangement.