Alex,
You don't really need to have your pre modified to have a ht bypass. Just use any input(except phono), just set the volime in the same spot all the time for ht. You also don't need the 2nd set of outputs, you can use a set of y connectors. You can always add them later if you want, but you can play with the subs for the stereo without spending any money on mod's. You will also need to change the settings in your ht receiver to no sub and the mains to large, so all the bass goes through the Tempest and the mains/sub.
Sure hope this helps
Jeff