I don't believe in fixing problems with cables and tweaks (And tubes). Its just too hard to get right. Think of it like buying an EQ with only 1 setting and you don't know what that setting is until you try it. Also, cables that make a big change or have that wow factor almost always reveal big weaknesses over time. Power Cords are even more difficult. Unlike speaker cables and interconnect, the effects of a powercord can be much more random from system to system.
There are a few things you may want to try. Vandersteen speakers are very detailed and revealing. I forget if the Quatros have this feature on not, but Vandersteens usually have an adjustment for treble and mids on the back of the crossover. If you back off the highs about 1/2 db, that might be enough to fix the issue. Any more than that, though, makes the speaker not sound right.
JPS makes a cable just for digital products. Its not that expensive either (About $300-$400 if I remember). I used one when I bought a cheap Sony DVD/SACD player to try DSD. Digital products that are not well designed tend to inject a lot of noise into the system. The Sony made all my other sources sound bad just because it was on and hooked up to the system. That said, you shouldn't have that type of problem with the McCormack; but it is a possibility. I would definitely recommend that you unplug anything in the system that you are not using when you are listening to the McCormack. I don't know what other components you have in the system but maybe its one of those.
If you haven't already done so, try the stock power cord. On my Ayre amps, they are the best sounding, by far. Also, if you haven't already done so, it try the Vandersteen biwired; not an internal biwire, but 2 separate runs of speaker cables. That makes a big difference. I couldn't believe it when I first tried it on my Vandersteens; its almost like a component upgrade. You don't have to get anything too expensive either.
I hope this info helps. One way or another, you should be able to deal with this issue. Also, (as you may have already guessed), I recommend you try other products besides power cords. I'm in a hurry and typing very fast, so if I was not clear on anything, just post and I'll see if I can help.