I just purchased an Onkyo Sr-304 and found my brothers old HTIB speakers(Panasonic) in the basement. I'm trying to save some cash for an upcoming trip which is why I can't little to nothing at the moment. The speakers I found were 4ohms(unsure of watts), but would it be safe to use on my receiver without damaging the receiver itself? I'm new to all of this, but I read on the back of the receiver that it said 6ohm impedance. What does that mean?
This means that you will work (overtax) the power supply transformer 50% more using 4 ohm speakers than 6 ohm speakers. It is especially true at louder volumes. At extended use with the 4 ohm speakers ( at higher listening levels) it will shorten the life of the receiver. How much, no idea.
6-8 ohms, but no higher as you will not get as much volume output with speakers higher than 8 ohms. BTW speaker resistance is not static, and goes up and down over the frequency range. So your 4 ohm speakers can go down to below 2 ohms at certain frequencies.