Digital filters on DACs are low pass filters that are more interesting as a matter of signal processing, even though they're (unsurprisingly) marketed to audiophiles (see the LG V30 cell phone). They are just different ways to cut off ultrasonic frequencies during signal reconstruction. All fast filters (whether minimum phase, linear phase, apodizing) do their work in the ultrasonic spectrum (22.05 KHz for 44.1KHz sampled digital audio), and the differences lie in how they govern ringing artifacts, which are also in the ultrasonic spectrum. So it's normal you can't hear any differences. You can google blind tests performed around these common filters. The slow filters, especially the AKM "Super slow filter" begin attenuating within the sonic spectrum. The effect is usually subtly softened highs. This can pair well with very bright headphones, like the HD800.