I have used the 501 for over 3 years and it is by a wide margin,the finest value in cartridges currently available.It does everything well.I replaced it by the Lyra Helikon,which is noticeably superior.The Helikon is faster,more detailed with more impact in the midbass.As a matter of fact,up to this point,I preferred the Helikon to every other cartridge I heard.
I then got the 901.The 901 posseses all the strengths of the Helikon and more.It projects a more expansive soundstage,is more harmonically complete with tighter bass and reveals more low level detail.The 901 is a significant step up from the 501 in every parameter of performance.The 901 may be double the price of the 501 ,but it is state of the art and I have personally not evaluated a better cartridge.