With my limited understanding of the subject, your observation makes perfect sense to me.
At the CD sample rate of 44.1 kHz, the fast filter must be really aggressive to allow a frequency response up to (around) 20 kHz, while still suppressing digital artifacts above the Nyquist frequency at 22.05 kHz. This gives a very sharp roll off, in contrast to the slow filter which rolls off more slowly.
At 176.4 kHz, the fast filter doesn't need to be at all as aggressive, because a flat frequency response all the way up to 80 kHz isn't critical. At higher sample rates, the fast and slow filters can have quite similar responses.
I've actually only had my CD9 for about a week, so I haven't played around with the filter settings yet. I want to get accustomed to the CD9 at default settings first.
Thanks for sharing your observations!
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