fireberd
The Behringer looks like a knockoff of the Ebtech hum eliminator.   I keep an Ebtech on hand in case its needed.

 
I actually bought the Behringer HD400 together with the Ebtech HD-2-XLR at the same time, in case one doesn't work. While the HD400 has been operating without any problem, the Ebtech gives a small buzz from day one... must be a faulty unit as the buzz came out from both channels.

Must be a bad unit as I've only used the ebtech a couple of times but it worked every time. 
 
One issue on XLR cables/connectors.   If the ground is connected to the ground lug on XLR connectors, on both ends, it can cause hum in some cases.  Some cables are made with the ground connected to the XLR connectors on both ends.  Ground not connected or only on one end is OK.  Got this from a retired TV broadcast engineer friend (who also had a consulting company for recording studios and video studios). 
 
 
 

Grounding in electronic is even more confusing then the meaning of ACT in SONAR.
 
We have several "grounds":
1) "ground" connector on power socket. It should be "real" ground (well connected with the earth). It should not be used to deliver elictricity. It is there just in case the power wire is by occasion touch metal parts we can touch as well, so "overcurrent" somehow switch electricity off and prevent our death. The theory is good, but the practice, especially in consumer area, is far from perfect. I was born in Russia, where outlets had no such connector.
2) "ground" (0) wire on power socket. That is used to deliver electricity (together with the power wire). It is normally "close" to the real ground, so for example we do not feel us bad in case we touch it. Well, if everything is correctly connected and it is not in use by some powerful device, in which case the wire own resistance produce significant potential shift. For that reason, it should not be connected with the "real" ground.
3) circuit "ground", as a reference for other (not power) signals inside the device. Depending on the power supply of the device, it can have virtually any potential compare to (1) or (2).
4) signal "ground" for transferring the signal (for example audio)
5) shield "ground", just to have the shield potential on some stable level.
 
So, for XLR it is quite logical:
* if both ends have the "ground" pin connected to the shield (inside device), it make no different how we connect it in the cable
* in all other cases of it's connection ((3) or (1)) we are looking for troubles (till at one end it is disconnected, inside the device or in the cable)
 
Theoretically, the same is valid for USB.
 
But there is one small problem... all "grounds" except (1) can be different between powered devices. The shield on all devices is (should be) connected such way, that the difference on it can not "kill" the device. It can create hum/noise or even switch off the device, but it should not kill it. From information in the Internet, audio circuits (at least in (semi)pro level) are also prepared for such "surprise". But for USB I am not so sure...
 
I must admit that my background in electronics has not helped me eliminate my hum, so I am waiting for the trafo
 
PS. Sorry for so long post.

Is there not something called a ferrite USB cable? 
 
I just google this and holy cow, I have a few of these already and didn't know it. I never new what that bump was for. Anyhow if you read up on it, it's a feature that helps reduce hum in audio related systems due to power transfer over the cable. USB buss powered audio interfaces are often terrible for background noise. A friend of mine bought the Focusrite 2i4 and had a bad hum. Later when I asked him he said he got rid of most of it by using a ferrite choke. 
 
Personally I think we are expecting to much from pour little usb. USB 3.0 was supposed to be better a supplying more power but seems more people are having trouble with USB 3.0 not working right than with 2.0. 

johnnyV
Is there not something called a ferrite USB cable? 

Interesting idea, thanks! I have not tried to use such cables in the setup, but I have them. Cost nothing to try while waiting for trafo.

Unfortunately no luck with ferrite... Absolutely no difference in my case (tested 2 cables in Piano setup).

So, HD-400 is effectively eliminating problematic loud noise in both setups.
 
Thanks for suggestions!
 
I am not sure I will get the second HD400 or try Neutrick for DP. I have to investigate from where some kind of "radio noise" still comes (it sounds like radio interferences on SW radio receiver, with changing frequency and tone), but that is not USB related and on the level of "you can not hear it in headphones when computer is working in the near". Ferrite is not helping with it (I really hoped, because ferrite is targeting RF...). But that is not a problem till I play some "ppp" classic and I do not need other devices "on" in that case.

Back in the mid 70's, I worked on "cleaning" a recording studio that was near an AM radio transmitter site.  The radio station was coming in on the AC power line, recorders, console, etc.  and headphones.  I was able to get the radio out, or at a low enough level that it didn't matter in everything except the headphone system.  I couldn't get it completely out of the headphones. 
 
Most of what I did was bypass capacitors. There was some shielding but the bypass caps fixed most of it.

Capacitor in parallel with some induction, connected to headphones... That was my first self build AM receiver.
 
The first time I have got that (the same) "noise" from DP, I have started to play Mendelssohn's Leads I have learned between soldering. That subtle effect of radio waves far behind piano music from XIX century adds some life to otherwise "perfectly dead" sound of DP
 
Probably it comes from bad "phone out"->"line in" cable I currently use. At place where I am, it is hard to receive radio. Also I am not getting that noise with other equipment (combinations).