Since you're running 5.0, you are taxing the amp section of your AVR more than you would if you were running 5.1 or 2.0.
But you don't seem to be playing at really high volume settings, so you're probably fine as you are.
One could attempt to calculate how much power you need, but in my estimation it's probably a futile endeavor. It just gets too complicated too quickly (having multiple speakers or different model, and who knows what the spectral density of the content is, not to mention that it's always changing...)
I doubt you need an external amp. I certainly wouldn't buy one. But if you want to, it might not be a bad idea to borrow one if you can (a friend? a dealer?). A two-channel amp running the fronts would take a lot of load off of your AVR. The problem with borrowing one and comparing is that you'd need to make sure it was a fair comparison, ie, level matched. Just the excitement of a new amp might make you think it sounds better, whether it really does or not, so you'd also have to account for psychological factors by quick switching if possible, and maybe even blind testing.
That's all fun for some people, but a total pain in the @ss for others. Which type are you?
