This has become a surprisingly heated issue in recent days. But, there are many misconceptions about negative feedback. The claim made by some manufacturers is that feedback is a "crutch" for bad design, and a way to make a bad amp measure well. In reality, the opposite is often true. It takes a lot more engineering to do a proper feedback amp, and higher quality transformers. Feedback is not a sign of a bad design, and can yield excellent results. There is a reason why vintage Scott, McIntosh, Marantz, and HK Citation amps are still revered -- careful circuit tuning, brilliantly good transformers, and well-executed negative feedback. When implemented properly, it leads to a less-colored sound, better damping, lower noise, and lower distortion. There are times when it can be omitted -- for example, with triode amplifiers driving *very* efficient speakers -- but (in my experience) adding a bit of corrective feedback can significantly improve the listening experience . 
I agree with the above and will add the following:
It is my belief that certain amplifiers sound really bad when they meet a load that, due to feedback, makes them misbehave. These are called conditionally stable amplifiers. Under many, but not all conditions they behave properly but meeting up with difficult loads they do strange and unpredictable things. This may remind you of some person you know who is conditionally stable.
By my experience only 10% of the amplifier designers in the high-end industry have any formal training in electrical engineering. Being an engineer of another stripe doesn't count, they don't teach topics like feedback or low noise design to non-electrical students. Stable feedback is difficult to implement even for those who know how. jon_10101 makes several good points. To quote him in reference to tube designs one needs "brilliantly good transformers" for as little as 12 dB of feedback. 50% of the time I spend on a complete design is for the transformers and the winding details. I wind all the prototypes myself and during the process I decide the best way to put down the wire to minimize the leakage inductance and parallel capacitance. For absolute stability the output transformer has to go out beyond 60 KHz and even then there are things I do to increase the stability. Marantz's Sid Smith used a tertiary winding (fancy name for another winding) for the high frequency feedback. I used this technique in the RM-9. Other techniques, like standing the output tubes on top of the output winding work well. This type of feedback is quite successful in the RM-200 because I have balanced feedback to the balanced inputs. BTW the RM-200 performs exactly the same in balanced and un-balanced mode. I wouldn't have it any other way. The RM-9 and 10 have somewhat traditional feedback to the input tube cathode in the RM-10 or grid of the diff amp in the RM-9.
Being a curious fellow I have measured many of the amps mentioned by jon_10101 and recorded the results
in three (going on 4) composition notebooks with results of tests I have made on amplifiers that have crossed my path. With 2-4 pages average that's 150 amps. Some I have owned, some my customers have owned and some people bring for me to evaluate. I can make over 75% of these amps oscillate into rather common loads. Sometimes the oscillations are "birdies" that only show at low frequencies due to the back EMF of cone woofers. Some will go into destructive oscillations with as little as 0.1 UF which can be the capacitance of many cables. I saw a respected solid-state amp go up in smoke with such a cable. Another amp went from drawing 50 watts from the wall to 500 watts when I put a very small 0.1 cap across the loutput. In cases like this (and there are many) the cap doesn't get hot at all but there are now 500 watts going in and nothing going out. About 30 seconds of this would likely cause a melt-down of the output transistors. These birdies and full power oscillations usually occur in the 20-50 KHz area so they cannot be heard but they might toast your tweeter. Birdies are little bursts of oscillation that appear on the main sine or music wave that I observe on my oscilloscope as I connect various loads. They are often rather small and look like a bird on a wire, hence the name. When they occur at low frequencies they show up repeatedly as the output crosses a specific voltage.
Of the brands mentioned by jon I found only the Marantz were up to my standards of stability, distortion and overload recovery. I could make all the others misbehave under various standards I have developed for my designs.
Here are my findings:
The HK Citation II had poor low frequency stability due to the feedback in the driver and the use of high capacitance coupling caps. Here is a perfect example where bigger caps are not better. Those who modify heed this warning: do not raisie or lower cap values unless you have the equipment to verify that your change is a good one. You could make the amp unstable in the low frequecy range.
HK went for ultra wide bandwidth but created other problems in the process. The amp takes several seconds to recover from a good clip. Given the age in which they were designed I imagine the designers didn't concern themselves with the clipping behavior believing they would never clip on the efficient speakers of the day. This poses a problem for current users who may indeed clip now and then. I designed for immediate recovery from overload as I can see 10-20 % clipping on a scope before it is audible to most listeners.
That's enough for now. Let me know if you want more on this topic.
