Several folks mentioned that they prefer the 901 whistle-style mouthpiece to the round one on the 501. On a whim I took a 901 mouthpice and tried putting it on my 501 manual model... it fits perfectly.
So if you don't like the round 501 tip, just replace it with one from a 901.
So if you don't like the round 501 tip, just replace it with one from a 901.