How do I use a sub with a tube pre for music only?

You would probably need a sub that can be set up for two different types/levels of input, like the REL subs. Typically you would pull the stereo signal from your pre from a second set of line outs, and the LFE signal from the LFE out of your pre/pro. I imagine there are other brands that can be set up this way, as well.

Assuming your running your pre pro through your tube pre for HT, run your sub through a 2nd set of line outs on your tube pre (assuming you have a 2nd set of line outs). I have my system set up that way. It works very well and gives me the best of both worlds.

The ACI subs also offer inputs for left and right channel and a separate single input for HT.

I have ACI Jaguars split off of a passive preamp into the left and right input channels of a Titan II sub. The HT processor sub output is plugged in to the "theater" input of the sub. Results are excellent with both.

This link is the ACI website section on subwoofer hookup variation:

http://www.audioc.com/library1/subsetupft.htm