Adding mass to a tonearm


I would like someone to explain to me why adding headshell weights doesn't really alter the mass of the tonearm that much when figuring cart vs tonearm compliance. I have a Denon DL-103r and I keep reading that's it's for high mass tonearms. I also hear that adding headshell weights doesn't really alter the mass. What gives?

I want to try a DIY on my Pioneer PL-530 turntable tonearm where I mask off the arm such that only the chrome arm on the headshell side is visible and spray it with Plastidip. This would seem to add mass and resonance control. If it doesn't work the I can just peel it off.  
last_lemming
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 bought various screws for my Reed 3p tonearm to add weight, because most vintage MM cartridges i have are too light, still usable, but the counterweight of the toneam is too close to the pivot, so those 12mm cartridge mounting thumb screws are 1.51g each (about 3g for pair) and made of the gold plated brass (non magnetic). Available here in various size and weight. Recommended!   
Soundsmith has  different headshell cartridge screws that changes mass.  Has anyone used these things?
New Pioneer PLX- 1000 comes with weights for both headshell  and rear of  arm .
Also smokes my 2K  TT.
SME III w/ 5 g wand comes with 2 weights for headshell each 2.2 g, so you can add extra mass of 4.4 g for less compliant cartridges. Convenient.