The arm length is wrong with the RB300. It needs a longer length than the Basik/Ittok config., so you need to modify the armboard, which is probably not easy to do and change it back if you have to. Google RB300 and LP12 and you will find links to the issue. The LP12 has the same arm set up as the axis I believe. With the LP12 you either have to modify the Linn armboard or purchase another board drilled for a Rega.
Rather than muck about with the Rega I would look for an Akita, then you don't have any of these problems.
BTW I have a RB300 on an LP12, so I had to live through this problem.