Here are the 44k test results for difference between slow & sharp filters. My test method is not ideal but it shows some differences. I generate the files in REW, burn to disc, play through my DAC (once with sharp filter, once with slow filter), record its balanced analog outputs to my Tascam SS-R1 recorder. Then analyze the WAV files from the recorder. The recorder certainly has its own FR, distortion, etc. so the absolute values don't mean much; what matter is the differences between the files.
Below: dark blue is "sharp", light blue/cyan is "slow". When I couldn't put both on the same screen, I always list sharp first, slow second.
First, frequency response. The slow filter rolls off earlier but the difference should not be audible (at least not to me).
Let's zoom in on that top octave.
These charts show no difference in distortion (sharp, then slow)
Group delay. As expected, a difference in high frequencies but it is so small as to be inaudible.
Let's zoom in on that high frequency group delay difference. The difference turns out to be above 20 kHz, so nothing to hear, here.
Now for the spec where I expected to see a difference: impulse response. The slow filter rises faster and decays slower, but not by much. I expected to see more difference here.
Here are the envelopes on these responses. I don't see much difference here. The slow has less ripple, but if you count bumps per division the ripple is at 22 kHz.
Here's the step response. Sharp has more pre-ripple, slow has more post-ripple. But the overall shape is virtually identical.
Finally, here are CSD plots -- no significant differences I can see.
This doesn't show much in the way of differences. But even that might be helpful to people interested in this subject.
