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another step to perfection.....geting rid of hum/buzz/noise

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Red Mastering:
Hi everyone!
I thought it could be good place to ask for a solution.
I have passive speakers in my system, my amp is a valve/tube power amp.
I have a few analog devices in my studio connected via patchbay.
Now the problem I have is a little hum/buzz I got in my monitors.
before I start, I don't know how to 'measure' this noise/hum,
please don't curse me for being not precise, explaining amount of noise for example,
so I will use non professional descriptions as - 'unnoticeable' or 'dead quiet' or sth. similar:)
if you advise me how to measure it - I'll do it
so the story of hum:
It's almost unnoticeable when only power amp is on (also it's much less when it's solid state amp instead of valve one - and it's obviously valve has more sensitive input),
I can't hear it until I put my ear into the speaker - so there's no problem, but obviously I need few more devices to work - then just power amp,
I used to have a hifi dac with rca outputs (just to inform my both power amps valve and solid state have rca hifi standard inputs) and this dac was completely 'dead quiet' when connected to power amp,
0 noise
I've noticed that hifi dac has a power supply not utilizing ground loop (that's what I think, but there's no way to find it- maybe I need to contact constructor?) - so maybe that's the answer, I dunno...
I noticed that every gear I have adds some minimal amount of noise to the system - I did checking using RME's digicheck software - it's brilliant;
Thermionic culture vulture LE is on top of the list - bypassed, then is TFPRO P9, P8 and diy SSL4k - the latter is dead quiet.
Since I plugged mytek 96dac into my system as a last monitoring dac, the hum/buzz noise became an irritating issue. When mytek was close to my PC (around 1 m) I could hear my HD in monitors...and many 'extra' noises, when I moved mytek and also plugged it in another power socket - it's much better. I can't hear the noise until I listen to classical music - then I can hear it and it makes me mad...
My room is ultra quiet, that's one of the reasons i can hear it.
It's not ground loop I think, as it's much more quiet then ground loop (I used to have ground loop issue before), it's i think mixture of EFM and hell knows what else all together
I also solder temp cables for checking new dacs and adc, so I used klotz cables - I noticed when I connected dac with belden (xxx1695 - sorry don't remember right now) - it was much less noise...
any ideas where to start and what to do ?
A week ago I went almost mad trying to find what cause the trouble and I unplugged every, EVERY cable in my studio...power, line, analog, digital...all of them:(
I switched off fridge, and all devices in kitchen:) - didn't help either..
apparently, when I moved power amp plug and dac plug into another power socket - it helped a bit (which is strange as all my devices were connected from same power socket- as they supposed to)
I thought about soldering proper (now it's good question - will it help?) power cables,
like this ;
http://diyaudioprojects.com/Power/diyMains/
but then read on Internet that it's all snake oil, etc...:
http://www.empiricalaudio.com/computer-audio/technical-papers/myths-and-snake-oil

any ideas how to approach this problem ?
I noticed this hum is not recorded when I go through analog chain,
it's only (!) monitoring issue

thanks for help in advance

Thomas W. Bethel:
Trouble shooting hum and buzz can get complicated very quickly.

The best way to proceed would be to pull out the input leads to the power amplifier, make up some adapters that short the tip and sleeve of the input jacks together (one for each channel) and have a listen to the speakers. If you still hear hum then it is in the amplifier itself. There maybe a hum adjustment on the power amp and you can rotate that for minimum hum. Some amplifiers used DC for the filaments and there is no hum adjustment, other manufactures just did not provide a hum adjustment and you amplifier maybe one of these. AC on the filaments can cause hum. Also leaky power supply caps can cause hum or a power supply that is not filtered correctly can cause hum. Once you have quieted the amplifier, replace the input leads and have a listen. You will have to work backwards from known to unknown until you find the source of the hum. You will know when you have identified the source of the hum because it will get louder when you hook the piece of equipment up.

A star ground system is good for a recording/mastering studio and you can find references for that on the WWW. In a star ground system basically all equipment is tied together to a central ground and all the connections between the equipment is  grounded at the patchbay but not tied to the grounds on the pieces of equipment. (Best to read about this solution in depth and follow it).

Maybe others have ideas as well.

Best of luck! :)

Red Mastering:
Hey Thomas!
thanks for input, I think I don't understand what you mean for shortening tip and sleeve?
I have xlr out of dac (mytek96) into rca power amp.
Amp itself gives almost nonexistent hum, there's something but you need to fit your ear into monitor and stop breathing for a second to be sure:)
When mytek is plugged (all equipment out of power socket - there's is hum/buzz described above.
Interesting when I use hifi dac, with rca out (I dunno if it got anything with this noise) - there's no noise, even less then just a minimal noise from amp (only power amp powered)
it's completely dead quiet;
It's not amp issue as I swapped 2 amps and the other one gives me same buzz, bit quieter as it's solid state amp. but it's there

Thomas W. Bethel:
What kind of inputs does your amplifier have? Phone, phono. XLR, or ????. Thanks!

Red Mastering:
amp has rca/phono inputs (-10dB hifi standard) - maybe that's the issue, as all my tools in audio chain are balanced, and all cabling - balanced, except power amp

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