This thread is so far gone but whatever here is my .02 cents. I don't think you can automatically assume the head unit caused the issues. Just because it's a knock-off brand doesn't mean its pure trash. First, clipping directly leads to speaker destruction. I am willing to bet you couldn't even hear your dash speakers clipping because they were ran off the HU and you had 3 other sets of speakers amplified. Secondly for the tweeters, did you install the crossover properly on them? From what your describing, it sounds like there is no crossover installed inline with these tweeters. This is exactly why the issue would repeat its self off of the head unit or the amplifier.
To who ever suggested going full active and 3 way and time alignment and 31 band EQS... really? Did you really suggest that as a serious option? No offense to the OP but he is light years away from going the route, let alone doing it properly. You don't learn to drive in a Ferrari, you start out in your moms Civic. The basics need to applied before going into an advanced set up like that... which even I bet 80% of the people on this forum couldn't set up properly.
Pretty much all aftermarket head units put out roughly 20watts per channel these days not 10. 10 is more in line of a factory system. They even make some that put out 50x4RMS. I've installed several and sound wonderful for an entry level set up.
I think there are several issues that need to be addressed with the install. One of which is that the OP is running 4 sets of speakers in a car. Is this is direct root of the problem? No, but I think we all should be here to educate and we all should know that 4 sets of speakers in a car is a bad idea. Your W6 should not be playing any mid-bass duties. It should be cut off around 80z. I think you should take a step from this build, read quite a bit and re-evaluate how you want to do this. You can accomplish the same level of volume with only one set of speakers, improve your staging, and clarity if done correctly.
And yes, go for the 4100, excellent unit that will offers many features as you build up the rest of your system around it.