The high range on my speakers are harsh...

How do things sound w/a good source. Phono or cd? You may want to change out your 16ga. spk. cables. (What are you using?) Hifi Cables UK sells the excellent QED 79 strand spk. cable. Warm sounding cable and extremely affordable. Not all metal domes are harsh/bright. It sounds like you have a source issue that needs to be addressed first and foremost. Your Rotel/Paradigm are really good pieces but they may not make the best pairing. Perhaps a Marantz or NAD int. amp may sound better w/your spks. Good Luck!

I'd bring the speakers to your friend's house too and see if the problem exists. If it doesn't you may be having a speaker cable or dirty connection problem somewhere in the chain. The fact that you re-connected your amp to your speakers and the problem decreased seems to point to some corrosion/dirt in the connnection. Maybe try some connection cleaner and see if that helps?

Brightness/harshness is often caused by poor speaker positioning in relation to the listener. The distance from the walls, the distance between the speakers, the distance from the speakers to the listener, speaker height, toe-in and tilt can all cause brightness. Depending on the size of your room, you may not be able to accomplish a perfect setup, but I've found that small things like reducing the amount of toe-in and or tilting the speaker back slightly can reduce harshness.

Here's a good summary of things to try.

http://www.gcaudio.com/resources/howtos/speakerplacement.html

If you had a CD player in the system, then it may be easier to find the problem. A CD player has fairly consistent sound quality, in comparison to a computer, or TV for the source. I'm not too knowledgeable about computers, but the ones I've used listening to something off of the net, can vary a lot in my experience.

A TV can be inconsistent with sound and picture also. When I hear a commercial on one channel, it may sound good. Hearing it on another channel can give good, or bad results. So a TV doesn't always help in finding a problem.

Then the computers themselves get moody, and change at times. So I'm thinking something more consistent like a CD player, or DVD player may help in this case.

You mentioned streaming YouTube. If you download something that sounds good to you using RealPlayer may help a little. When you get this sound problem, play that video that you downloaded to RealPlayer, and see if it sounds OK. This isn't as good as a standalone CD player for consistency, though. But still, may give you more of an idea.

Using the output from a computer using these (streaming) sources you mentioned don't have the sound quality, like a CD does. All of my computers own sound outputs, vary a lot too. Even if I play a CD in them.

Your speaker and amp combination, is probably really accentuating any small problem.

A poor connection is a possibility, but you are using two different sources (TV and computer), with separate cables for each. So, I'm thinking the odds are slim that both are making a poor connection, since this problem existed, since you bought the system.

I guess what I'm still saying is a CD player, DVD player, may help a lot in finding if it's your system, or the inconsistent (computer/TV) sources you're using, causing your problems. If you don't have a CD player, see if you could get one for testing. This is the only other thing I can think of, since its sound quality way more reliable, especially using the same good sounding discs. Other than this, I'm kinda out of ideas. I hope you find an answer to this.