Hypex N core module

Did you have experience with amplifier based on Hypex - NCore Technologies  
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I’ve been reading for decades that negative feedback in traditional linear amps should be avoided since it (sorry, no pun intended) negatively affects its sonic performance, especially in the upper frequencies. I’ve taken this as the gospel truth ever since I first learned about the use of feedback in amps.

First you need a bit more clarification on just what "feedback" are you talking about, "global feedback" or Local feedback"? As both are very different.
"local" is a good thing if the output stage is designed right with minimum problems. "Global" is also ok if used sparingly and not use to correct big problems.

Then there's "feed forward" that I've seen in Rouge M120 monoblocks.

Cheers George
They heard that right. But had they been in a position to add 60dB, well then, suddenly they would have been confronted with a sound that is little short of magical."  
Bruno is spot on with this. The usual rule of thumb is that very low amounts (under 4 db or so) is not harmful, but more than that is a problem; after about 20 db or so things start to settle down. 60db hasn't been practical until the introduction of class D (since gain is developed in an entirely different way); so I have no argument with this; my prior comments should be limited to traditional amps where making the kind of gain is impractical.
I use the Acoustic Imaginery Atsah monos and also do not experience any high frequency issues or soundstaging issues. The Ncore 1200, imo, is extremely revealing, neutral and transparent and will take on the character of the source equipment, cables and material being played.

As such, careful attention needs to be paid for the Ncores to sing and for its potential to be maximised (just like any high end amp). It has been quite an interesting journey and I have spent considerable efforts matching the appropriate cables with the Ncores and cannot be happier with how they sound at this point. Class D amps are very sensitive to power i.e. power conditioning and power cords and so I will suggest these be properly looked into and matched before any critical evaluation of Class D is made. 
At the risk of making the least contribution to this thread and over generalizing the discussion, my somewhat limited experience with hearing class D amps at various shows and friend's homes has had one thing in common. That quality, almost irrespective of the downstream or upstream gear, was that I always felt I was listening to the instruments as opposed to the music as a whole. Whether its the ultra revealing nature of the technology or the choice of the associated gear, I can't tell but I was consistently blown away by the sound for the first few minutes followed with my brain going into overload with the amount of detail I was hearing and ultimately felt exhausted with the demand. Some may consider this experience the holy grail of audiophilia but for me it felt like I was watching the trees as opposed to enjoying the forest scenery. Perhaps crude analogy but the best I can do.
Class D amps are very sensitive to power i.e.
 Yes they have one of the most dirtiest power supplies made, SMP's
That's why you'll see many caring Class-D manufacturers giving them more quieter linear supplies, just to null out one of the problems of Class-D amplification.

Cheers George