There are differences in audio low pass filters between different Amiga models. It can't be ever entirely disabled. I think Amiga 1200 filter affects the sound

There are differences in audio low pass filters between different Amiga models. It can't be ever entirely disabled. I think Amiga 1200 filter affects the sound the least. IIRC, AGA chipset (Amiga 1200, 4000) is also capable of up to ~56 kHz sample rate. OCS/ECS is just up to 28 kHz.