Ken,
For your current configuration, the routing you described makes perfect sense. If you use Hauptwerk Advanced and more audio channels, then you may wish to create an Aux mix channel pair or a pair which could be used to drive the subwoofer and possibly an outboard reverb. Or, you could route only pipes below a certain note number and route these to a channel pair to drive you subwoofer. In this case you really should have speakers to use with the subwoofer to handle the upper harmonics of the bass pipes.
The Echo Audiofire 12 and Mackie HR824 monitors are a great foundation.
Joe Hardy