I have an SMMC 4 from Soundsmith on my BM 5000 and I can recommend it, with a new cart. you will get the sound that a new cart. is able to produce. If your old cart. is sounding good and you like the sound, continue using it, it may last a couple of years more, but it will go bad as all of these old cartridges, so buy a spare from Soundsmith. The sound from vinyl is not only produced of the cartridge but also from the source (your vinyl) then the cart. and after that the innards of your BM 5500, preamp, amp and then speakers, if your vinyl is like new and all the rest is in pristine or very good shape, you will get the best sound, but if any of the other items are bad or mediocre your sound will be thereafter.
Saludos amigo
Beosound 3000, BL 4000, BL 8000, BG 2404,BG 5000, BG CD50, Beocord 5000, BM 901, BM 2400, BM 4000, BV S45, BV 3702. There is nothing we cannot do, but a lot of things we don't want to do!!