Keep in mind that slew rate and damping factor are not like the world land speed record, where engineering goes into pushing the envelope higher and higher. Prudent amplifier designers design for performance, not specmanship. Is more better? Only to a point. The necessary slew rate is directly proportional to the highest frequency the amplifier needs to produce and to its maximum output voltage. Thus, the slew rate for a full-range or high-frequency amplifier should always be high enough to permit full output voltage swing beyond 20 kHz. But an amp can have an excessively high slew rate, too, allowing it to pick up RFI on the inputs and either pass some of it on to the speakers or demodulate it and produce unwanted noise and jabber in the audio. Similarly, the output impedance of an amp is usually due to the output network, which decouples RF that comes in from the speaker cables. In the real world, though, your speaker cable resistances are usually going to totally overwhelm the effect of the output impedance on damping the speaker cone. Any effective damping factor (that's taking into account both the amp's output Z and the speaker cable resistance) of, say, 100 or higher will provide you with optimum bass control--as long as you have good speakers. There are good ways and there are bad ways to get amazingly high slew rate and damping factor in an amplifier design. Take out the ultrasonic filtering and you can get a super-high slew rate spec, but the amp would be vulnerable to the RF effects I mentioned above. Take out the output network and you could have a damping factor of maybe 20,000, but you'd let RF into the circuitry and you might make the amp unstable with certain reactive speaker loads. Beware specmanship.
