Does 21g seem high for effective tonearm mass?

21 grams is medium mass. It will work better than all those skinny-mini 10 gram arms, especially for low-compliance cartridges.

In general higher mass arms control the cartridge instead of the other way around - am I right, Raul ??

Fleib, it's all relative. My highest compliance cartridge is a Denon DL-S1 at 14 cu - the FR-64fx I've got works well with this cartridge mounted in an Orsonic headshell and the small counterweight. Great cartridge for choral and chamber music, etc. When I want more body, I switch out to an Ortofon Synergy SPU and the W250 counterweight, which takes the effective mass up to 30-35 g, (more like an FR-64s). This combination also works well - sounds like a Koetsu with iron cajones !!

For flexibility in compliance matching, it really helps to have a removable headshell arm with light and heavy counterweights - the FR-64fx is still one of the best !

If the cartridge compliance is low enough to cause the arm to move rather than the cantilever to move when the groove undulates back and forth, you get no coil velocity in the magnetic field and hence no signal. This happens ALL THE TIME with low compliance cartridges and too light tonearms on low frequency undulations. Hence light tonearms can sound lightweight.

Same goes for unipivots with too low a moment of inertia in the azimuthal axis. Because the groove is offset (below) the CG of the arm / cartridge, the groove will rotate the ARM back and forth about the unipivot point, rather than move the cantilever. That is what I meant by having the arm control the cartridge instead of the other way around. Gimbaled arms and linear trackers don't have it's extra monkey motion and have fewer problems reproducing low bass as a result.