Grado Cartridges

[bikerhifinut]

I was messing about getting a nice old PL12D fettled up for my Great nephew who is a very sophisticated youngster in asmuch as he's got the LP bug off his dad and me.
So I thought, (the boys 16 next month so this is a present to upgrade the excellent nick but obviously limited BSR MP60 we started him off with) I'll try a different cartridge to the venerable Shure M75EDit had (new jico stylus out of my toybox).
I remembered I had a little used grado precision black in the stash that had been on a Rega Planar3 I bought a bit back off greg of this parish, beautiful condition. However the Grado hummed like a buzzsaw from the AC motor, apparently a known issue so it was replaced with a Rega carbon to get the kid going.
So I've got this grado, and i put it on the Pioneer and even with a metal platter and a pressed steel chassis screening the motor it hummed very obtrusively.
The annoying thing is aside from this godawful hum issue, its a bloody nice cartridge for £100 last time i checked. Really nice for a MM.
Now heres the killer, I put it on my Michell Gyro, DC motor no Hum fields anywhere and it still thrummed like a bumble bee.
I was puzzled and then thought hmm theres the CD player and the Rega Cursa3 preamp below but separated by a good couple of feet. switched CD off and the hum dropped substantially, Then I swapped over to my Valve CF pre where the Psu is a lot further away and when I turned the Cursa off.............. blessed silence.
Now the point of this is, would you spend good money on a cartridge, no matter how good it is meant to be that didnt have the most basic hum shielding, NONE of my other carts display this issue on ANY turntable.
Frankly I think that renders the Grado prestige range "Unfit for purpose" and if I'd bought it new I'd be demanding my money back.

What would you do?

Andy