Mad Scientist IC’s

So in another thread, I described having issues with audible speaker hum when I connected my Jolida dac directly to my NuForce STA-200 amp via a pair of Schroeder-Dueland IC’s.  Lots of discussion regarding the high gain of the NuForce and the output impedance of the dac.  A few weeks ago, I purchased a pair of Mad Scientist HAC+ IC’s from MrTriode (who btw is a great guy to deal with), which after proper break-in, replaced the Schroeder-Dueland IC’s. First off, they sound amazingly detailed and three dimensional in my system.  Second, the hum disappeared altogether.  Going back to the Schroeder IC’s reproduced the hum.  So I’m curious— the Mad Scientist’s use carbon fiber; the Dueland’s, tinned copper.  What difference in the materials used eliminated the hum?
Of all the many things it could be you assume its this one in particular. Why?
Speakers hum with the Schroeder- Dueland IC’s; they don’t hum with the Mad Scientist IC’s.  No other changes to the system.  
It's obviously the different RCA plugs used by each; it couldn't be anything else.....................
@renisnceman - WRT the hum
1. is the hum a 50/60 Hz mains hum? or is it higher or lower in frequency
2. is the hum a constant level?
3. or does it get louder as the volume is increased?
4. what speaker cables are you using?

Which model Jolida DAC do you have?

I see the STA200 is a power amp
- so I assume you are controlling the volume via the DAC - correct?

The microphonic effect - e.g. The cable from my turntable suffers a microphonic effect.
- i.e. I can hear sound if I tap the cable with my fingernail
- does this happen in your case?
- this can cause hum if in close proximity to other cables

Is the cable in question near a power cable or a component power supply?

One last thing to try - using a piece of wire, touch one end to the neutral side of the RCA socket the cable is plugged into and the other end to a "common ground point"
- does the hum go away?
- if yes, it may be because of the grounding design of the attached components and the geometry of one of the cables highlights this anomaly

Also, Cables can cause issues within attached components and sometimes it is another cable (e.g. speaker cables) in the system that causes the problem
- answers to all questions above will help get to the "root cause" of this problem

Regards - Steve