I do not know 12" so cannot comment but in general heavy mass arms recommended are from fidelity research, or to find, sme - all vintage. Again low compliance carts will work in jelco you can also add damping fluid to provide even more compatibility (I have never liked the fluid though) but if you check other forums people claim the design of Jelco was not heavy arm from ground up but rather added compatibility. Again I'm running carts that require mid mass so I'm not an expert if you ask about heavy arms. I'm planning to sell my arm as a part of rig reduction so let me know PM if you interested in.