What cause tweeters hiss ?

lso the 15 years old XLR may not have a good shielding against WIFi ... perhaps.

Many of these guesses are interesting ....except, everyone seems to forget that the OP stated that the volume of the integrated amp is at Zero - which is AFTER all these cables and will block all signals and signal-mixed noise.  For clarity i placed a flow chart of a typical signal flow in my original reply, waaaayyyyy above.

I do, but for $100.00 I will get rid of it for you.

Cash only...next Thursday at midnight at the gas station...be there or prepare for more NOISE...

You will find that the hiss reduces in volume as you upgrade and tweak for noise: primarily cables along with a good conditioner. Hold the hairspray! 😉

RFI radiated from someware or through the power being supplied can sound like hiss on the output. It is above human hearing so you the hiss might sound as a harmonic. RFI power filter may do the trick. Good grounding will help. I use power isolation transformer with RFI filtering after a Furman RFI power strip. I have no hiss.  

....except, everyone seems to forget that the OP stated that the volume of the integrated amp is at Zero

The volume control is at its lowest setting. That may not be off but attenuated enough to not hear music playing.