It’s a difficult problem, because the unique Decca cartridge design gives very different horizontal and vertical compliances, so the tonearm mass is a compromise between the two. I think that the tonearm needs to be very rigid, have damping, and be of medium mass, around 10-15g roughly (the Decca International tonearm was 9g, and had silicon damping). Many s-shaped Technics tonearms, provided they've been modified with fluid damping, would be suitable, as long as a heavy headshell isn't used (i.e. nothing over 10g).
Without arm damping, the Decca (now known as London brand) cartridges apparently have trackability problems due to an under-damped cantilever. The ideal tonearm, of course, would be one of the superb electronically-damped tonearm turntables from Sony, JVC or Denon, since they’ll adapt to any cartridge.
Another member did put up the late Garrott Brothers’ advice about their Decca cartridge modifications in a previous thread about Decca cartridges. The Garrotts were the world specialists on improving Decca cartridges, and said that standard Decca cartridges varied a lot in how well they were damped – one of their mods was to improve that, by modifying how the cantilever damping was attached, so that they were optimally-damped. Because of the flimsy mounting bracket and body, they also recommended Plasticine (or similar modelling clay) damping between the headshell and mounting bracket, to prevent resonances in the mounting bracket and body. They allegedly also treated the cantilevers to prevent rust, according to someone who worked with them for a while!
So the mandatory thing is a damped tonearm, whether by fluid or electronic damping, to achieve the best sound quality from such a unique design, and a very rigid tonearm.
