Ratpack,
I am not really an expert in loud listening:)
You have shown very convincingly that by applying certain numbers and assumptions one can arrive to an absurdly high number - 2400 W per channel. That's the SuperDome class acoustic.
I see you have the same Onkyo 703 as I do. I think it's a very capable and powerful receiver. However it might be not so for you.
Here's what I would suggest. Pick an inexpensive multi-meter and measure the average power coming into your speaker. You can measure the voltage if you connect it to the speaker's posts or the current if you connect it in series with the speaker. Then you will have an idea how much power the receiver puts out into the speaker at your listening level.
Now, granted it will be the average power. If you have a scope, you can make the same measurements for the instantaneous power. If not, well, you can apply some reasonable multipliers. At the end, I believe, it will make the decision of whether upgrade just a bit more sound
