With the Decca, adding mass to the headshell seems to be the most effective, in my experience.
I have observed this with the modified Rabco SL-8E on my Garrard 301, and with a homebuilt Well Tempered clone that was on my modified AR belt-drive. On both of them, adding mass to the headshell improved bass definition and extension, and imaging. I am guessing that since the Decca throws a lot of energy back into the arm, added mass would tend to reduce the effect of that energy, otherwise more of it would get various vibration modes going in the arm itself. Just my guess...
I have observed this with the modified Rabco SL-8E on my Garrard 301, and with a homebuilt Well Tempered clone that was on my modified AR belt-drive. On both of them, adding mass to the headshell improved bass definition and extension, and imaging. I am guessing that since the Decca throws a lot of energy back into the arm, added mass would tend to reduce the effect of that energy, otherwise more of it would get various vibration modes going in the arm itself. Just my guess...