[Steve]

You want the Ohm rating of the speakers to match the Ohm rating of the receiver and vice versa. If you have a receiver rated for 6-16 Ohms (seems to be popular), then you want the impedance rating of your speakers to fall within that range. It's not a good idea to try to play 4 Ohm speakers on a receiver rated for 6-16 Ohms. You risk burning up the receiver.

When it comes to wattage ratings it's a little bit different. When a speaker is rated for 150 watts, you have to look at whether that's a peak rating or a continual rating (expressed as RMS). If it's a peak rating then the speaker can handle 150 watts, but only for a few seconds at a time. If it's rated at 150 watts RMS then it can handle 150 watts continuously. Your better speaker brands will show the wattage rating for their speakers as RMS rather than peak.

It's Ok if the wattage ratings of your speakers and receiver don't match. If your speakers are rated for 100 watts RMS, but your receiver is rated for 150 watts per channel (and can actually push that much-more on that later), then you're still Ok as long as you're somewhat careful. The wattage rating of your receiver means that's how much power the receiver can provide, not necessarily how much power it provides continually. In a nutshell, the power supplied by your receiver is directly related to how loud you have the volume set. For comfortable listening levels you may only be pushing somewhere between 10-20 watts per channel. This is not going to harm your speaker. But if you put in a good action movie and really crank the volume then you'll be pushing quite a bit more power. This is where having plenty of power comes in. Every time you double the power sent to your speakers you're only adding 3 dB of volume. So if you increase from 20 watts to 40 watts going to your speaker you're only increasing the volume by 3 dB. To increase by another 3 dB means you must double the power again to 80 watts and the cycle continues.

It's often Ok to push more power to your speakers than what they're rated for, within reason of course. The key is you want clean power. It's a good idea for your receiver to have more power in reserve than you'll actually use. This is known as headroom. By not driving it to its limit you're not putting a strain on it and you don't run as much risk of clipping or introducing distortion, and clipping and distortion will wreak havoc on your speakers. If your speakers are rated for 100 watts and your receiver is rated for 100 watts per channel and you have it turned up as loud as it can go then you're likely getting clipping and/or distortion (because you're running it to its limit) and there's a very good chance of damaging your speakers, even though the speakers and receiver are both rated for 100 watts. On the other hand if your speakers are rated for 100 watts and your receiver is rated for 150 watts per channel and you have the volume turned up just as loud and you're pulling 100 watts per channel, you won't do any damage because the power is clean and there's no distortion or clipping. This is where you may even be able to put 110 or 120 watts to your speakers without damage. Again the key is clean power.

Lastly be aware that manufacturers often overstate the wattage ratings of their receivers. It's common for a receiver's specs to say it has 90-100 watts per channel with all channels driven, but when these receivers are actually tested they usually measure somewhere in the 40-60 watt range. Some even lower. Certain lines like HK, Marantz, and Pioneer Elite tend to be quite accurate while others like Onkyo and Pioneer (non-Elite) are often considerably inaccurate. This doesn't mean they're bad units. It just means they don't have the power they say they do.