Shelter 501 mkii then what now

rebuilt Shelter 901 with ruby cantilever from Soundsmith

well you know I was thinking of getting a retip from
soundsmith can anyone tell me what I might expect in the
way of improvements with the ruby upgrade thanks for you
help so far

If it needs re-tipping you really have nothing to lose. You end up with a great sounding cartridge for $250 - $350. It sounds different, but not necessarily better or worse. Hard for me to describe with the passing of time since I have heard it.

tracking ability improves drastically to the point of producing substantially less surface noise. Ruby rings substantially less and has less resonance vs. aluminium or boron.
I've no doubt that 901 may sound better, but you won't loose a dime if you just upgrade current.