Assuming that the output on your preamp is volume controlled then that would be perfect. If it isn't you will be able to tell very quickly as the volume on the sub won't change with the volume on the preamp.
You can hook it up to either the left or right input on the SA-1. Whichever one you don't use, hookup your AVR to the other input. The only challenge you might have is level matching the output of the sub to both the AVR and the preamp, but if you have enough adjustment on the AVR it should work fine.
Edit: realized that you will need to use the left and right output on your preamp to the left and right on the SA-1, so you won't be able to have them both hooked up at the same time without adding a switch box.