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.
I have Rega Planar 3 with Grado Reference Platinum and use a sheet of Mu-metal from lab kit 110 purchased from Magnetic Shielding Corporation in Illinois. A bit of a pain to cut and fit but it does the job and stops the hum.
thanks for the info everyone! dave, i owe you one...
i mounted a grado reference master last night. there is no hum from it whatsoever.
the sonic improvements are vast. soundstage many times deeper and wider. bass is no longer over emphasized which may only be because of it's relationship to the mids and highs, which are much clearer/more transparent and extended; but in any case bass is also much tighter, and no "boominess".
everything resolves faster.
the single most striking thing may be the amazing sound of vocals (cause the mids are so wonderful?), especially in relation to each other during harmonies. the harmonies on abbey road are beautiful. i have the mfsl cat stevens teaser and the firecat. it never blew me away and i always wondered why it's a TAS pick. last night i found out why (holy harmonies, vocal fans). johnny hartman w/trane moved BEYOND the sublime, and ella's cole porter was NOT just one of those things.
cannonball's solo coming in after coltrane on all blues is 100 times as lyrical as it sounded before (it never made me think of desmond before, even for a second. the experience hendrix remake of are you experienced sounds like a different mix entirely. sorry for running on, but all this before its even broken in is pretty exciting. anyway, thanks again kids...
Jim,makes me want to upgrade to the Master also! WOW! I'm glad you are so thrilled with your music.Yes,the Grado's are very fine cartridges.Enjoy!!
P.S. The Grado's need about a good 30 hours to really show their stuff.You aint seen nothin yet!