If the sub has a line-level connection (aka RCA) you can patch the sub in using the second dual outputs from your pre (use the single-ended RCA outputs like you would to hook up a second amp). This will allow you to use the volume control to control both the main amp and the sub.
If you only have one set of outputs, you could get a high-quality Y-adapter to put on the preamp outputs to drive both the main amp and sub off of one set of outputs. I think Cardas and Dayton both make a set. There are likely to be other brands as well.
