After I have got second "instrument" with the same problem, it is time to find a solution...
 
1) TC VoiceLive Play GTX connected to computer with USB, connected with Kawai CN43 from headphone output (1/8'' stereo) to line input (2xTS)
2) Roland TD-11K connected to XENYX QX1202USB (2xTS), which in turn is connected to computer with USB.
 
Both combinations are working fine for me (there are in different rooms, so different computers and the rest) till I connect either of the instruments (e-drums or DP) to computer with USB. The result is the same in both cases: significant noise/hum in middle/hi frequency (not 50Hz).
 
I have tested several USB cables, no difference. I have tested other USB ports (throw hub, without, etc.), there is a difference, but it is still not acceptable. Notebook on place of any of 2 PCs (especially on accu) works fine.
 
Since I conclude I am getting some ground loop (PC->usb->mixer/VliceLive->DP/e-drum->usb->PC) and I do not like an idea to cut ground on USB cable (I have seen many smoked devices at work after using such "solutions"), I am thinking about some trafos:
 
VoiceLive has balances XLR outputs (mic level), so I think about XLR to TS:
http://www.thomann.de/gb/neutrik_na2mjtx.htm
 
TD-11K has unbalanced TS (also mic level? I am forces to use mixer preamp inputs to get on normal one), so the idea is to "save a bit" with:
http://www.thomann.de/gb/behringer_microhd_hd400.htm
 
Does someone had the same/solved such problem?
 
EDIT:
 Solution: Behringer HD 400 helps in both cases.

I've got good result with the Behringer HD400. I used to hear some digital noise when I turn up the volume of my audio interface to the max - when there is no audio playing. With the HD400, the noise is completely gone. My setup may not be the same as yours, but the HD400 is cheap enough to give it a try.

Thanks for the comment. I think I will try HD400 first.
 
As a piano player, I am more sensitive to the noise on DP. That is why I have thought about Neutrik there, but may be I can somehow test HD400 for piano as well.
 
You are right, my dinner yesterday was more expensive then all these devices together... my wife has enjoyed the dinner, while she is still skeptical about my hobby 

Azslow3,
Your original post is a bit confusing in terms of signal chain.  Just for clarity, is it like this?
[TD-11K Drums]---{Unbal. Audio}--->[XENYX Mixer]---------{USB}--->[Computer]
[Kawai Piano]-------{Unbal. Audio}--->[VoiceLive Play GTX]---{USB}--->[Computer]
 
Also, you have have two, separated setups except for the computer (which is the common point).  The instrument setups use different wall outlets, right?.  This can cause a ground loop.
 
But overall, I think the issue stems from the USB connections.  I experienced the same issue in the past with a Waldorf Blofeld synthesizer. It was horribly bad.  I think I ended up swapping the USB cable for a MIDI cable as the USB-audio feature was a bit crappy anyway.  After the swap, the hum/noise was gone. Regardless, I didn't like the Blofeld so I sold it!
 
The Behringer HD400 seems like a good solution to try since it is relatively cheap. Buy it at a brick-n-mortar store that will allow a hassle-free return if you're not satisfied.  If it helps clean up some digital noise, as tomixornot has mentioned, then I may try that as well in between a Roland V-Synth (noisy bugger!) and an Alesis Ai3 interface.
 
EDIT: Since you don't want to break the ground connection in your USB cables, then the Behringer HD400 (or similar device) will safely break the ground connection in your audio cables.  Essentially the result will be the same.  You'll need two HD400's, one between each stereo-audio path from your instruments to your interfaces.

My previous setup :
Quad Capture (unbalanced) -> HD400 (unbalanced) -> Behringer Truth monitors (active).
 
My current setup :
Quad Capture (unbalanced) -->>
-> Nad Amp (driving a JBL Sub woofer)
-> Nad Amp Rec out (unbalanced) -> HD400 (unbalanced) -> Behringer Truth monitors.
 
There is no noise from the Quad Capture headphones, only when Quad Capture -> Truth direct. I wonder if using a pair of balanced 1/4" cable would solve the problem too..?

I had a hum problem in my Octa-Capture, until I used the correct "Line Out" rather than the headphone out for my studio monitors.
 
I use TRS plugs for the monitor speakers.

If it really is a ground loop, the way I always handle it is to use one of those 3 prongs to 2 prongs plug converters (is this an American thing?) on various plugs in the signal chain and check to see where the hum gets louder or softer based on the change.
 
Sometimes it gets MUCH worse, sometimes much better.
 
As long as there is a path to ground, you should be ok, but you don't want to lift the ground if there is only one path to ground. So for example, if you have a power strip plugged into the wall and everything plugged into the power strip, don't lift the ground at the wall. Lift it for the items being plugged into the power strip.
 
ymmv.
 

2:43AM
Azslow3,
Your original post is a bit confusing in terms of signal chain.  Just for clarity, is it like this?
[TD-11K Drums]---{Unbal. Audio}--->[XENYX Mixer]---------{USB}--->[Computer]
[Kawai Piano]-------{Unbal. Audio}--->[VoiceLive Play GTX]---{USB}--->[Computer]
 
Also, you have have two, separated setups except for the computer (which is the common point).  The instrument setups use different wall outlets, right?.  This can cause a ground loop.

The second setup is:
VoiceLive (headphone output)->Kawai (unbalanced line input)
VoiceLive (USB)-> PC(USB)
And the problem comes when there is (in addition)
Kawai (USB) -> PC(USB)
 
I have already mentioned that 2 setups are on 2 different computers in 2 different rooms. In each room all equipment is connected to the same outlet. So, there is no usually mentioned "power ground" loop.
 

tomixornot
There is no noise from the Quad Capture headphones, only when Quad Capture -> Truth direct. I wonder if using a pair of balanced 1/4" cable would solve the problem too..?

My (stage) monitor is connected from XENYX with TRS (balanced). In case I use TS (unbalanced), I get more noise. But the effect is negligible compare to what I get in the case of USB loop. With TRS, the background noise level is the same as with disconnected boxes (cheap stage monitors are not noiseless, but they are cheap and big, that is what I wanted for drums ).
 
I have ordered HD400, I will report the results with both setups once I get it.
 

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

fireberd
I had a hum problem in my Octa-Capture, until I used the correct "Line Out" rather than the headphone out for my studio monitors.

The hum from "incorrect" line out is on the level I do not care. The connection is there just to sing along with piano without reconnecting my headphones every time (I keep all "surrounding equipment" off when not in use, so the headphones are always connected to the piano).
 

gswitz
If it really is a ground loop, the way I always handle it is to use one of those 3 prongs to 2 prongs plug converters (is this an American thing?) on various plugs in the signal chain and check to see where the hum gets louder or softer based on the change.
 
Sometimes it gets MUCH worse, sometimes much better.

That is some kind of "USB ground loop". Most probably incorrect design of the related circuits. Why someone has decides electrically connect USB ground to something? I remember times when network interfaces was grounding cables, burning motherboards within seconds.
 
As I understand, 3to2 plug is something I have found by Neutrik. I will try that in case Behringer does not work in my case (Neutrik is 2 times more expensive, next step can be Palmer, which is again 1.5 in $$$ compare to Neutrik).