I think you're fine with a rewired RB-300. I would agree with a counterweight mod - a number of them you can install yourself. I had a rewired RB-300 with Origin Live mod which I preferred the the Linn Ittok that came with my LP-12. Specifically, I thought the Ittok had a spurious brightness compared to the Rega. Alternatively the Ittok is livelier than the Rega. I haven't had exprience with the Michell Technoweight but it's had a number of good reviews.
The OEM Moth arms are actually the RB-250 which is the down market arm from the RB-300 you already have.
The OEM Moth arms are actually the RB-250 which is the down market arm from the RB-300 you already have.