What cause tweeters hiss ?

If the volume is turned a long way up a low level hiss will usually be audible if you put your ear close to the tweeter.  The hiss is a noise product of the amplifier but will be at a very low level relative to that of any signal that is connected (don't connect a signal at that volume level).  The level of the hiss can be defined as its level relative to that of the signal - signal to noise, S/N, ratio.  In a well-designed amplifier the noise level will be less than 1/1000th of the signal level.

All amplifiers generate noise, but if the amp is functioning correctly it will always be inaudible from the listening chair at any sensible volume level.

@auronthas 

I am very surprised, assuming you are saying that when you select CD input on your amp/preamp and turn the volume up with no signal you do not hear hiss; whereas it is audible when you select other inputs and do the same.

I am not clear how the selection of a different input can alter the noise generated by the (power)amp.

Re your initial post, I don't think hiss is likely to be caused by grounding issues; they invariably lead to hum.

And, frankly, DACs can cause all kinds of issues.  I regard them as the black art of digital sound reproduction and its Achilles heel.

Interesting.

So premium interconnect can worsen SQ as well as improve it!