All amps add a bit of distortion, but when you are reading power measurements, which is the most common way for manufacturers to give their power claims they will say: 75 watts @ x%THD where x is your THD level for that power.
If you look here:
http://hometheatermag.com/receivers/denon_avr-4810ci_av_receiver/index4.html
You will see a receiver like the Denon AVR-4310CI measures between .001% and about 2% distortion levels on the test bench with two speakers driven between 1 and over 175 watts.
So, what is the THD? What is the actual power?
The reality is that you don't notice THD unless it is pretty high, but the manufacturers will use ridiculous numbers (10% sometimes) THD to increase their claimed wattage and will make those claims based upon one channel driven. A quality manufacturer typically makes the claim based upon two channels driven.
Very few do all channels driven THD tests, but Home Theater Magazine does and publishes the results - but at the end of the day, the test is not relevant to normal listening... typically.
The ohm load on the receiver also matters with the test - typically it will be done at 8ohms, though some will make claims based upon 4 ohms, or even 2 ohm loads.
Depending on the context where you are trying to get THD levels from, and why it matters, is when this all comes into play.
A receiver claiming 120 watts per channel, with 10%THD is likely sitting on top of a piece of junk receiver which could hurt your speakers if you try to drive them loud. A quality manufacturer, like Denon, Pioneer, or Yamaha will prevent more than a couple of percent of distortion before shutting down the receiver to help protect your speakers.
The most common way to damage speakers is not by overdriving them with to powerful an amp, but turning up a weak amp way to much and under driving the speakers.