Dear Frank: My experiences are that the headshell/cartridge " connection " must be " intimate " and tight with nothing between each other.
Everytime I try something between cartridge/headshell sound changes and IMHO not for the better but this is only my personal experience where can/could be different from other people ones.
Anyway, that "after-market " VTA for the Rega arms is very usefull and convenient.
Regards and enjoy the music,
Raul.