I will work from the bottom of your post on up...
Wow. I was wondering the same thing about the larger variances in dB from say Surround R and Surround L.
Room treatments and acoustics will make a difference, but that still seems like a lot.
I've seen sub distance estimates being off due to echo and wave cancellation, etc, again based off of room acoustics.
As for using an SPL meter, I haven't done it, but I have been told and have read that it is the best way to do it. I will let others that have actually done it speak about that point.
Separate amps (from decent brands) will pretty much always provide a better quality sound, and yes will boost output volume.
I will let someone else also talk about the watts. One thing that I will say is that if you start feeding a quality speaker a distorted signal or have too much receiver clipping even, at high volumes, this can damage any speaker.
There are a lot of mathematical numbers around what a watt means in comparison to sound levels and such, but like I said, I will let someone else speak to that.
