Y ou say that your pre has no sub out, but does it have a line out other than the main line out? If yes, I'd use that. If no, simply split each channel of the main line out w/a Y splitter. That way you'll have a stereo input at your sub. I've done this and it works fine.
Subwoofer connection to a tube preamp
What is the best way to connect a subwoofer to a tube preamp that has no sub out? I have an Audio Electronic Supply (Cary)AE-3DJH preamp and would like to connect a sub to it. My sub has line-in (R&L) and line-out. However, the line out has a high pass filter which cuts-off at 80 HZ. Would it be better to use a single RCA-Y or a Stereo RCA-Y to provide a line input to the sub or would it be better to use the line in and out on the sub. My main speakers are Klipsch La Scalas and are being driven by a Cary Rocket 88R tube amp. The system response is lacking in bass so I would like to supplement it with a sub. Any suggestions? Thanks, Larry