I don't have the technical knowledge to describe why, but here are some posts on another forum.
Mr. V. wrote on his forum:
It's not complicated. Feedback is time distortion. Vandersteen's are Time and Phase Correct.
Ralph Karsten (Atmasphere) gave a more detailed explanation on feedback:
There are several reasons feedback has a bad rap in high end audio.
The main reason has to do with where the feedback is applied. In most tube amps its the cathode of the input tube. In many solid state amps is the input of a transistor that is part of the differential input of the amp.
In both cases these devices aren't linear. As a result, the feedback signal gets distorted prior to being able to do its job! So of course its going to add distortion as a result.
The distortion added is IMD and higher ordered harmonics. A lot of the innate distortion of the circuit does get suppressed.
If the resulting distortion profile lacks the 2nd and 3rd harmonics in sufficient amount, the higher orders will be audible as they are not masked by the lower orders. This will cause harshness and brightness. We've all heard it!
There are two solutions. One approach is to make the feedback node as linear as possible. Thus the feedback isn't as distorted as it would be otherwise. Here are two examples of that- the feedback used in an opamp, where the feedback is entirely external to the opamp itself, instead being mixed with the incoming audio thru a restive divider network. The other way is to use a really linear tube like a 12AX7 section to receive the feedback. In this way you might not have to use a lot of feedback and the circuit might still sound quite good. The Dynaco ST35 is a good example of that in practice. The ST70 is not.
The other solution is to use a lot of feedback. Like over 35dB; this can overcome the non-linearity of the feedback node. The problem here is that you have several things that can stop you in the circuit. One is that the circuit might not have the gain or bandwidth to support the feedback. If this happens distortion will rise above a certain frequency. Distortion rising with frequency isn't a good thing- its a clue that the amp might sound bright and harsh although the bass might be just fine (since the feedback is supported at the lower frequency).
The second problem you can have with this approach is circuit stability. Because of frequency poles in the circuit, feedback at audio frequencies might be negative, while at some ultrasonic frequency it can become positive due to phase shift associated with the frequency pole(s). Sources of a frequency pole might be a coupling capacitor, a coupling transformer, Miller effects in the active devices (tube or transistor), stray capacitance and the like. If the feedback loop isn't properly designed, the amp could go into oscillation if a signal of high enough frequency can get through the feedback loop and phase shifted enough to be positive feedback.
Class D amplifiers offer a way around both of these problems. They are particularly easy to develop the gain/bandwidth product that is needed to support a constant +35dB or more of feedback at audio frequencies. If the amp is of the self-oscillating variety, you can add so much feedback that the amp goes into oscillation, but then the oscillation is used as the switching frequency. The feedback network can be designed so that the amp will always find the same switching frequency, killing two birds with one stone.
A large amount of feedback can compensate for phase shift that might otherwise be present if no feedback is used and bandwidth is limited. It also allows the amplifier to reject AC noise problems. So there's a lot of advantage to this if you can pull it off.
Mind you, we've been building zero feedback amplifiers for nearly 50 years so I'm not saying this idly. A zero feedback amplifier will not have the dreaded rising distortion with frequency and if designed properly, will not exhibit harshness and brightness since the higher ordered harmonics are masked. So this can be a very musical amplifier, but also can be load sensitive, so the choice of speaker used can be critical to really getting this kind of amp to show off its stuff.
A very high feedback amp gets you to the same place if designed properly and has the advantage of being usable on a wider range of speaker loads.
HTH
Bob