NFB lowers output impedance. If amp has 10x higher gain before applying of NFB then feedback will lower output impedance tenfold. We call it 20dB feedback. As a result deep NFB some amplifiers end up with output impedance of single milliohms. There is no need for that, since inductor in series with the woofer is about 0.1 ohm, while the fact that speaker's internal impedance is mostly resistive limits even more. When speaker's coil moves in response to signal it also generates response (back EMF) that works like brake that limits coil movement. This brake has damping effect on the membrane.
DF=1000 looks good on paper but it doesn't make big sound difference. Deep NFB used to achieve it, on the other hand does, making sound bright and unpleasant. Highly praised Atmasphere amps have output impedance in high ohms (DF=1).
Some speakers work well with low output impedance amps while others, (overdamped) sound great with tubes, but sane level of DF is perhaps 100.
It is very tempting to improve specifications with NFB because it doesn't cost anything. If you see fantastic spects in cheap amp you know that something has to give - it is sound.
DF=1000 looks good on paper but it doesn't make big sound difference. Deep NFB used to achieve it, on the other hand does, making sound bright and unpleasant. Highly praised Atmasphere amps have output impedance in high ohms (DF=1).
Some speakers work well with low output impedance amps while others, (overdamped) sound great with tubes, but sane level of DF is perhaps 100.
It is very tempting to improve specifications with NFB because it doesn't cost anything. If you see fantastic spects in cheap amp you know that something has to give - it is sound.

