The question was asked, "How can you damage speakers by under-powering?"
I am more in the Pro Audio realm with Bands, etc.
The concept of damaging your speakers by under-powering them is because you will be tempted to push what little power you have,
to get the result that you want, in doing so many people will drive their power amps into clipping and fry the voice coil.
However as long as you are responsible, know your limitations and do not drive your amp to clipping then you will be fine.
How far you would have to go to do this with a home receiver I do not know,
does the Pioneer 1020 or any receiver for that matter actually have a "Clip" light?
If not, there in lies a lot of the danger, in that you could be clipping the amp section and not even know it.
Pioneer would be best to answer this theoretical question.
You would definitely want to match the speakers "Ohms" to the amp as correctly as possible.
You can find out about all of the Theory and practical usage at http://www.prosoundweb.com/
Please do not ask questions in their Forums...It is not for or about Home Use Receivers.
Still there is a lot of information available in regards to Speakers, Crossovers, Amps, and Equalization.
Theory that you can carry over into your home setting.
If you really want to go that deep.