What cause tweeters hiss ?

I had the same problem it tends to drive u crazy. Whatever outlet your using in the house isolate it from everything else. Then get a copper ground rod and hammer it into the ground just outside the outlet you’ve isolated . And that’s it hiss be gone, good luck 

Tahquitz, you are saying that you had a 4,000+ Hz ground loop?

Elaborate please.

Possibly he needs to shield his chassis, but that's very different.

I have owned many expensive and moderately priced amps over the years and none are dead silent. If you can't hear from your listening chair don't worry about it. If you have to know, then you have to isolate what is the source. So the only way to do that is to disconnect the entire system. Then you connect the amp and speakers and listen. Then keep adding 1 piece of equipment at a time until you isolate the offending device or cable. Once isolated, then you can work on a solution. You may even discover that your system is fine & there is something on your homes electrical grid that is causing the hiss.

It seems to me that if the noise goes away when you unplug the DAC, then the problem is in the DAC or its source.
 

Try floating the ground with a cheater plug and see if it dissappear. Generally, the only component that you have to ground is the preamplifier.