Almost assuredly his Pioneer receiver is the 'weak' link if he wants to push the volume. He doesn't state his exact model but M60s can take much more than any mid range receiver can dish out.
Also, I disagree that M60s are "bright". I only hear "bright" if I play bright source material. The term is subjective though for most, so it's not an argument, I just personally would not describe them that way.
Putting aside safe listening levels, I would agree that the receiver is likely clipping when it hits its limits. This results in audible distortion and as mentioned, can damage your gear. If you push it too much, the speaker might fail which makes people think it's the weak link. However, it is actually the clipped audio signal that was damaging the speaker. It's not the speakers fault.
If you need to go louder and remain clean, you are on the right track. Invest in more power. Just remember that once you hit those very loud levels, it can take exponentially much more power to get even just a slight bit louder. Eventually you hit a point where adding another 100+ watts to the total is not even noticeable.
You might try adding a really good sub to the mix first though. There is nothing like having your shirt vibrate to satisfy the loudness craving without even having to touch the volume dial. That won't solve the clipping at existing levels though. You still need more power for that.
In any case, do what makes you happy to listen to music. That is the end goal here.
