With the 676, I'd suspect you only have one x-over choice, and we probably don't even know what that is.
The first thing you should check is if you have your speakers set to small or large? All of those you listed should be set to small, otherwise they will have frequencies below what they are capable of sent to them and this sounds like what is happening. That is not distortion from clipping, that is break-up from the driver trying to reproduce frequencies below what it is physically capable of.
Distortion does not come from too much power, it comes from not enough power, but that doesn't sound like your issue. The 150w rating is a maximum rating and it doesn't have any bearing on this particular issue, as far as I can tell.