Adding mass to a tonearm

First, I don't believe that "S" arms are necessarily lower mass.  That will be a function of the material the arm was formed from, how thick the walls, the length, and so on.  If fact for a given length, a straight arm with offset headshell will likely be lighter than an S arm with the same effective length when other factors are equal.

However, to experiment, you could take a small ball of Blu-tak or plumber's putty weighing a gram or two (use your VTF scale) and affix that to the headshell, rebalance the arm, and see if this offers any sonic/tracking improvement.  If so, keep adding weight until it becomes worse, then reduce it to the optimal.  If no improvement was observed, then forget about it.

Note if that does give an improvement you should be able to determine how much mass could be added for a more permanent solution.

Raul, and last_lemming, maybe I didn't make myself clear.  My suggestion to experiment with bits of clay to add mass was not meant as a permanent condition.  It was merely an easier way to find out if, and how much, added mass would improve the performance of the cartridge.  It went without saying if say a 2 or 3 gram increase was beneficial, then find a headshell that much heavier.

Otherwise, experimenting with a number of different headshells could get expensive and would be very time consuming, given consideration to mounting and alignment for each trial.  That was why I included the thought of "a more permanent solution".