I'm going to have to go against prevailing opinion here. It's always been my experience that the biggest differences in sound are made by the transducers (cartridges and speakers). This is not a fashionable opinion, but it's what I've found to be true. So I'd go with a cartridge before upgrading the preamp. I have no experience with any of the carts you mentioned, but can say that the Shelter 501 II that I recently bought is absolutely superb in every way that matters to me. And it's within your budget.
-Bob
-Bob