As someone else wrote, adding mass to the headshell most certainly DOES increase the effective mass of the tonearm, by a factor roughly equal to the added mass, in grams. (As you move down the tonearm toward the pivot, the effect of adding mass at any point on effective mass lessens proportionately.) Adding mass to the headshell will also cause you to need to move the counter-weight further back away from the pivot, in order to counter-balance the added mass and achieve the same VTF. Doing that ALSO will increase the effective mass of the tonearm, by a factor equal to the square of the change in distance from the pivot to the center of mass of the CW, times the mass of the CW.
From what I have been able to learn without owning a DL103 or 103R, there is almost no limit to the effective mass that those cartridges might "like". One of my friends uses a home-made tonearm with his DL103 which appears to have an eff mass of nearly 50g! (Probably I am exaggerating, but it's mass-ive.) I would suggest just experimenting with added mass at the headshell (and correspondingly re-balancing your tonearm) until you reach an effective mass that seems to be optimal for your cartridge. Then use a test LP to guesstimate the resonant frequency.
From what I have been able to learn without owning a DL103 or 103R, there is almost no limit to the effective mass that those cartridges might "like". One of my friends uses a home-made tonearm with his DL103 which appears to have an eff mass of nearly 50g! (Probably I am exaggerating, but it's mass-ive.) I would suggest just experimenting with added mass at the headshell (and correspondingly re-balancing your tonearm) until you reach an effective mass that seems to be optimal for your cartridge. Then use a test LP to guesstimate the resonant frequency.