By the way, 28 khz is even higher than the maximum frequency limit the human ear can hear. 50 khz is a nonsense since the human can't make any difference between a music at 20 khz and 44,1khz
That is actually not true. It's true the ear can't hear a sound higher than about 20khz, BUT sampling frequence is NOT the frequency of the sound you put out. For example a square wave is made of a ground frequency (the frequency you hear) which is a sinus wave and then you have overtones that add up to that sine wave to create a square wave. It's what's make different instruments sounds different, so the higher frequency you can output, the more overtones can be combined to create the sound. One rule is that you MUST have TWICE the sampling speed to be able to read a wave correctly. Since a wave is made of two parts (high and low if you think of it in a simple way), then you need to get atlest two samples for every frequency span you want to read or else you can't read both the high and low part. so to be able to sample a 20kHz audio wave you need at a minimun 40khz sample rate, but that would mean you get one sample at the low part and one at the high part, so you would ONLY get a triangle wave even if you had a square wave to begin with. This is very easy to see on a oscilloscope.
One common rule is to use 10 times oversampling to get something that looks like the original wave, so if you want to sample a squarewave (which is the hardest one since it's so sharp edges) who is 20khz, then you need to have a sample rate of 200khz!
So even a CD at 44100Hz isn't so great. compare it with a audio DVD at 96khz and you will hear a big difference. It has ofcourse to do with the frequency of the instrument you listen to. A high frequency instrument sounds worse on a CD than a low frequence instrument.
If you listen at a sample on the STE at 25khz and 50khz you will hear a big difference and sample rate is just sample rate so there isn't any difference between an amiga and an atari then you compare the rates. They are both 8 bits to, so it will sound the same (if you forget about filters and such stuff)
From what I have read, the Lance tracker should output true 50khz mods in just 30% sample rate.
If you want to hear a STE game with mod music, just play Obsession

to playback an 8 bits 44,1Khz, you need an A1200 with a 68040+FPU. This gives an idea about the 50Khz on STE.
This has to do with the Paula chip that needs a VGA resolution to play at that rate. It has nothing to do with the amiga itself. you need an Amiga with ECS chipset and then you can have 56khz samples, even on a Amiga 500
ST / STFM / STE / Mega STE / Falcon / TT030 / Portfolio / 2600 / 7800 / Jaguar / 600xl / 130xe