Advice for amp with in wall speakers.

In addition to my main system, I want to have a second system that plays throughout the house and outside. I would like advice on a second power amp that would drive six or eight speakers. My understanding is that this causes an impedence problem. Is this correct? What do I need to look for in an amp? I plan on using the preamp from my main system, directed into a box that will switch the in wall speakers. How much power do I need, and do I need an amp that will go below 4 ohms. Thanks in advance for any help.
I have an Adcom 5006 which is a custom installation amp that is designed to do this. 50 watts into each of six channels. The channels can be daisy chained or bridged in many configurations. Check it out on adcoms web site.
You may also want to check out and look at their impedence matching volume controls. With these you can use any 2 channel amp or receiver to run all your in wall speakers. They sell a distribution plate that will allow you to use up to 10 pairs of speakers. Good luck.