So if all watts are not created equal, how can I know the difference?
A watt is a watt... you have to look between the lines of the marketing brochure, though... 'cos 110WPC looks more impressive. My Marantz is the same: 110wpc rms... 2 channels driven. Its estimated that with 7+ channel running, I'm maybe getting 70wpc. Some specs even "show boat" by giving you what only a single channel will produce if nothing else is connected... Listen to much in Mono these days?
If you're not familiar with it, this is a cool tool to help you out:
Peak SPL Calculator
It's also helpful to keep in mind that you have to double your power output for every 3dB increase in SPL.
So say you have my 85dB sensitivity speakers:
1w=85dB (@ 1M)
2w=88dB
4w=91dB
...and so forth.
Or, say you have some crazy Klipsch or Tekton Speakers...
assuming they are accurate measurements.
1w=98dB
2w=101dB
4w=104dB
8w=107dB... which clears Movie Theater Reference Level
peaks.
Mind, Normal conversation is ~60dB, and a rock concert is ~120dB. Threshold for hearing damage is ~85dB, and threshold of pain is ~120-140.
I would say, compare what you want to buy, to what you have. And consider if you are going to upgrade your speakers. If you fall in love with a speaker that is low-efficiency, and notoriously power hungry, you would want much higher amplification. Conversely, speakers that are more efficient and say they only require a min of 20W RMS should run quite nicely on 50-70W.
Hope this helps!