There is a purpose for this brickwall low-pass filter. Without this filter aliases of high freqencies will fold into audible band. Filter has to pass 20kHz but reject 44kHz carrier - it cannot be done with even group delay (linear phase) Bessel like filters (no matter how many poles). Result of using non-Bessel filter is wrong summing of harmonics and altered sound.
When you say upsampling you probably mean oversampling. Upsampling is particular case of oversampling with non-integer ratio and asynchronous reclocking.
When you say upsampling you probably mean oversampling. Upsampling is particular case of oversampling with non-integer ratio and asynchronous reclocking.