Effective mass is a quantity based on the distribution of the mass of the tonearm, from pivot to headshell. The closer to the headshell you add mass, the greater the effect to increase effective mass. So the most efficient place to add mass if you only want to increase effective mass is at or on the headshell. I am guessing that by wrapping the WT tonearm in plumber's tape you gained an additional sonic benefit related to damping or changing the resonant behavior of the tonearm. As you wind the tape in toward the pivot, you are doing less and less to change effective mass. But I am no big fan of the WT tonearms.
aserejeje, You can get an approximation of the amount of signal current you can get out of a LOMC cartridge by simply dividing the signal voltage output by the internal resistance. So you can see that the Denon DL103 is not a good candidate for current drive. (Do the calculation for some other LOMCs, and you can see my point.) Yes, it's that 40 ohm internal resistance.