Adding mass to a tonearm

Kombi, I think you can find the consensus of opinions by reading the above posts.  If you want to save money, of course you can just add mass to the headshell.  (There's no law of man or audio against doing that.) Raul and others mentioned the possible drawbacks of going that route, but it certainly "works", and it's certainly a cheap experiment. But first of all, I would listen to what you've got until you have a very firm grip on how it sounds and whether you really feel the need to do anything at all.  Then when and if you do decide you need higher effective mass, you will be able to sense whether adding mass takes you in the right direction.

I tend to agree with Pryso.  Look at the Technics EPA250 tonearm: The optional arm wands with lowest effective mass are all straight pipes with a tiny fixed headshell.  The arm wand with high-ish effective mass is an S-shaped pipe with provision for optional headshells.  This suggests that Technics realized the effect of pipe shape on eff mass. However, I also agree with Raul.  Adding mass to the headshell is not "the best" way to increase effective mass.  Changing the headshell for a heavier one would be the simplest alternative way to go but as Raul also said, this will also change the "sound" by a bit.  Another route is to add some mass evenly across the length of the arm wand, by wrapping it with tape or heat-shrink or whatever.  But ALL of these methods will possibly change the sound.

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.