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.

is this a typo?:

"Hiss sound disappear only until power cord is disconnected."

When you connect the power cord the noise goes away.  When you disconnect the power cord the noise returns?

something is not quite clear here.  If the amplifier has the volume all the way down, no sound ought to come from the inputs. It MUST be in the amplifier stage(s) following the volume control. Typically a circuit is switches --> volume --> preamp line stage --> power amp/current amp.

So i suspect this is not electronics hiss at all, but some kind of high frequency interference that comes from whatever device (Denefrips????) is connected.  Note i have  Denefrips and it is very, very quiet.  It is also isolated at its USB input.  That said i also isolate my bridge and power them independently.

not a ground loop. ground loops occur at 60 Hz (or 50 Hz in most parts of the world). Not tweeter frequencies. A ground loop is essentially a "signal" caused by the difference between two different ground reference points being amplified.

Can you explain this again from the beginning? We are all missing something

When you refer to "power off", what precisely are you powering off?  The accuphase?  The denefrips?  Pls be very precise.

Is there no preamp in this system? I see no mention of it. Or I missed it.

What about with no input to the [preamp?  magic box? whatever is before the volume] whatsoever?  This is the "null hypothesis" and reveals the amp / preamp hiss.

I forgot to ask. Is this s new problem? If so what changed?

Yes, i guessed that English might be your second language, and thanks for the clarification.  Your English is vastly better than my mastery of most languages!

Your situation is a mystery.  Since the volume being down (essentially off) should black any noise coming from the Denefrips, the ONLY answer that makes possible sense is some high frequency interaction. As noted the frequency is totally wrong for grounding, plus, you seem to have it grounded correctly.

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.

Unspecified, but i will assume that this is the XLRs (balanced cables) between your DAC and integrated amp? Therefore analog.

Yes, unless there is some major defect in the XLR cable, this really makes very little sense.  I have seen, very occasionally, that when a ground is lifted or weak, rather than simple hum (60 Hz, 120 for rectified power) there can be a complex noise that includes higher frequency stuff. So maybe, just maybe the XLR ground is damaged (broken).  This raises a simple way to triangulate is it one or both tweeters? If BOTH then it would have to be both XLRs - which is unlikely.

I don't think you have found the true culprit yet.

I have absolutely no idea under what mechanism bad XLR cables would cause hiss.  At most, better shielded cables would reduce noise.

Nor do I.  Buit the entire question as posed is a mystery.