ok so if i understand correctly, digital filtering is used for obvious differences in sampling frequencies where an analog filter cannot change its cutoff frequency (well, a switched cap can...
Correct.
The various types of digital filters exist because consumers 'demand' these options and because manufacturers either come up with different filters themselves to be 'different'. Arguably a steep linear phase filter is the most correct one that fits the sampling theorem best.
One can also create steep filters with post ringing and enough attenuation but with lmore post ringing.
Even here there are different implementations with different amounts of taps.
Drawbacks of linear phase are latency (not an issue with listening to music) and the fear of the dreaded 'pre-ringing'.
This pre-ringing happens outside of the audible range. Most people believe all frequencies 'pre-ring' and this 'degrades' sound and they would rather see digital versions of the first CD Player filters (with no pre-ringing but massive post ringing.
Then there are the 'I want square-waves' reproduced correctly believers and the 'impulse' guys that base their belief on the reproduction of an illegal pulse (one that does not exist in music and is never recorded).
Usually these filters have a slow roll-off, so filter less outside of the pass-band and thus have more ultrasonic crap.
There are ones with and without (very short) pre-ringing and post ringing. Looks nice with square waves and impulse plots but alas these signals do not exist in any recordings and are for tech guys that want to evaluate filter aspects.
Some folks believe the stored bit levels should be reproduced and that this represents the actual audio info. They forget what the actual samples are and that samples are points in time but are reproduced in sample and hold (which differs) and that these filters do not comply to the theorem.
There are plenty of plots found on the web that show the plots
@Calexico wants to see. Google 'imaging test'.
There you can see which type of filter produces how much 'garbage'.
Archimago often shows these plots.
They are enlightening and is what
@Calexico 'demands' to see from Amir otherwise he cannot see how the DACs perform in the 20kHz to 100kHz range which he (and some others) believes is important for perceived quality.
Whether or not the present ultrasonic 'garbage' may or may not result in sound degradation (never in improved fidelity) depends on many factors.
In most cases, using analog amplifiers there is no penalty and the ultrasonic garbage is simply there but not reproduced by transducers (headphones/speakers) and not heard by humans either.