I can't speak to the VPI 3D arm, but regarding the counterweight issue, I agree with Stringreen. At least on my 12" SME arm, the counterweight is very heavy and is as close to the pivot as is possible with the design. My AirTight Supreme cartridge weighs 12.5 g which is the ideal weight for this arm and allows the heaviest SME counterweight to slide very close to the pivot. The cartridge was developed in part on this arm.
I believe the relevant issue is moment of inertia. The closer the counterweight is to the pivot, the less inertia and the quicker the arm can respond to movement as the stylus tracks the groove. And I presume the less the cantilever has to work to overcome the inertia of the arm, lessoning distortion as the stylus remains more in the center of the groove.
There is also the issue of resonance within the structure that supports the counterweight. If the counterweight is far from the pivot, I think the support is more likely to resonate, all else being equal.
This does not address the issue of resonance in the main armtube that was discussed up the thread, which is perhaps the main design goal of the 3D arm. It will be very interesting to see how this arm performs.
I believe the relevant issue is moment of inertia. The closer the counterweight is to the pivot, the less inertia and the quicker the arm can respond to movement as the stylus tracks the groove. And I presume the less the cantilever has to work to overcome the inertia of the arm, lessoning distortion as the stylus remains more in the center of the groove.
There is also the issue of resonance within the structure that supports the counterweight. If the counterweight is far from the pivot, I think the support is more likely to resonate, all else being equal.
This does not address the issue of resonance in the main armtube that was discussed up the thread, which is perhaps the main design goal of the 3D arm. It will be very interesting to see how this arm performs.