cartridge for rega planar 3 - 800 range

i want to upgrade from a grado gold. any advise on the right cart for my rega planar 3/2000? i am using an audiosonics ba150 tube amp, melos tube pre/phono, kef series 105.2 spkrs, grado rs1 headphones, chang lightspeed power conditioner. i have had people tell me a better cart won't improvesound much. i do not know what to think. am considering a grado reference master. many thanks.