You don't say, but I assume this problem happens with both pairs of speakers. Although the tweeter can be on the harsh side with your speakers, your case sounds extreme. If all 4 speakers have the same problem, the issue will most likely be with another component. Here's a list of some things you may want to consider.
Your amp may not have enough power. I know that's the most obvious choice, but I would be surprised if that's what it was. I've heard those speakers many times run from a receiver with no problems.
Your amp may be defective. It is, after all, brand new. If you have a different pair of speakers, try them.
Since your amp is for HT use, you may want to check your settings. You may be running the signal through some type of weird processing feature.
You may have defective cables. That's fairly common. Try swapping them with something else, if you have.
I don't know what you are using for a source, but if it has any type of gain control, turn it down.
Make sure you didn't plug your source into a phono input. (Yes, I know you probably know better.)
Last, go over the speakers really well. The screws on those things do work themselves loose over time. That's something you need to check anyway, even if your problem lies elsewhere.
Your amp may not have enough power. I know that's the most obvious choice, but I would be surprised if that's what it was. I've heard those speakers many times run from a receiver with no problems.
Your amp may be defective. It is, after all, brand new. If you have a different pair of speakers, try them.
Since your amp is for HT use, you may want to check your settings. You may be running the signal through some type of weird processing feature.
You may have defective cables. That's fairly common. Try swapping them with something else, if you have.
I don't know what you are using for a source, but if it has any type of gain control, turn it down.
Make sure you didn't plug your source into a phono input. (Yes, I know you probably know better.)
Last, go over the speakers really well. The screws on those things do work themselves loose over time. That's something you need to check anyway, even if your problem lies elsewhere.