Koetsu cartridges - what arms work?

I have heard many fine systems utilizing Koetsu cartridges, but, I cannot say to what extent the particular tonearm contributed to, or detracted from, the sound I heard.  A local dealer that is particularly fond of Koetsu cartridges likes the Ortofon RS-309D tonearm (12" tonearm that can accommodate a SPU-type headshell).   

I have been using a Koetsu black on a Fidelity research FR64 arm and highly delighted for the last 20 years.

 Racedoc, did you actually make quantitative measurements of skating force? Or are you reporting your observation of what the tonearm does on a grooveless LP? In any case the skating force will be a function of the friction of the stylus in the groove and the angle of tracking error. There’s no getting away from the calculation. I do question the importance of skating force generated by by tracking angle error.  The possible benefit of an underhung tonearm like the viv float is that there is no headshell offset angle which also generates a skating force that is additive with tracking angle error, at points on the LP where the cantilever is not tangent to the groove. And by the way examining skating force on a grooveless LP is fraught with sources for error. There is much less friction on a grooveless LP for one thing. 

Finally, I am not sure I understand your first sentence where you say that the skating force changes direction with the viv float. Can you explain further?

Fidelity Research FR64s - excellent combination with my Urushi Vermilion.

Addendum.  Upon further thought, I do see your point about the skating force changing direction.  When the stylus is riding in the outer grooves moving toward the one instant when it is tangent to the groove, the tracking angle error is getting narrower until it disappears momentarily at the point of tangency.  Then the stylus keeps on riding toward the inner grooves, which is opening up the tracking angle error in a different direction until it reaches another maximum at the innermost groove.  This would also change the direction of the skating force.  My remaining point would only be that by my calculation, the max tracking angle error for underhung tonearms like the RS-A1 and the Viv is much greater than for conventional overhung tonearms.  Yet, with my RS-A1, I don't hear it at all.  In fact, the RS-A1 paints a smoother more coherent picture across the surface of an LP than do most overhung tonearms.  That is what makes me wonder if the real "problem" with most tonearms vis a vis skating force is created by the headshell offset angle.