If you had something like klipsh, I'd say that 100wpc is great. Not for a pair of 86db speakers. With your equations used, you would need at least 400 watts to even come close to what an efficient 92db speaker would produce. Most klipsch are even more efficient than that.
Secondly, THX requires 110db and THX ultra is 112b. I think it may be a little excessive but these were chosen for a reason.
Finally, one think not mentioned directly here is a sub. I agree with Sean that the bass will be robbing you the most. However, most people cross the bass over at 80hz. If you are doing that, 100wpc is more than enough. Keep in mind though that the sub needs to be up to the task. My guess is that you should be looking at a 400+ watt sub. As a general (very general) guide, producing bass usually uses up about 75% of the wattage required in an amp. The higher you cross the system over, the easier it is on your amp and the harder it is on the sub amp.............
Secondly, THX requires 110db and THX ultra is 112b. I think it may be a little excessive but these were chosen for a reason.
Finally, one think not mentioned directly here is a sub. I agree with Sean that the bass will be robbing you the most. However, most people cross the bass over at 80hz. If you are doing that, 100wpc is more than enough. Keep in mind though that the sub needs to be up to the task. My guess is that you should be looking at a 400+ watt sub. As a general (very general) guide, producing bass usually uses up about 75% of the wattage required in an amp. The higher you cross the system over, the easier it is on your amp and the harder it is on the sub amp.............