JMW 9 effective mass?

9.5 is correct. LOL You are right Audiofiel. I am not willing to accept reality. I may be at the limitations of my cartridge, tonearm, preamp. I wonder if you can mount a Triplanar on a Scout, probably not worth it. Maybethat would not even do it.

I used a VPI JMW 10 with a Koetsu Jade for many years so perhaps I can help here.
First of all, a compliance of 15 is not a low compliance cartridge. It is squarely in the medium catagory. Low compliance is usually considered < 10 such as a Koetsu and denon 103 which have a compliance of 5.
Have you actually listened to this combo and found it wanting or are your concerns coming from some sort of math model? The math formula's are just guidelines. Their are many combinations that are poor math matches that sound great in reality.
IMHO, the arm would work fine with your cartridge. Adding the 3gm aluminum weight is probably a good idea to get a more ideal mathematical match. Another way to do this is to buy brass screws for the cartridge which will add which will add 3-5 gms over the typical aluminum screws.
My Koetsu sounded pretty good in the JMW 10. The stone bodied Koetsu's are 4-5 gms heavier then their wood bodied "cousins" so I was adding weight without doing anyhting extra.
Regarding damping fluid, with a true low compliance cartridge, IMHO, no damping fluid is needed and that's I had mine setup. Usually, VPI recomends damping fluid only for high compliance cartridges such as Grado's or older Clearaudio's(new?). You might want to try it with no damping fluid at first and then add a drop or two and see if you like it better.
Lastly, the Triplaner is a great tonearm but it is not ideal for true low compliance cardtridges. It is on the low end of the medium mass arms with an effective mass of around 10.5 gms. That being said people have used it with a modified Denon 103 with good success. It would be great with your cartridge but I don't think you would be hearing everything your paying for until the cartridge and table got upgraded.
Enjoy the listening and the process. Hope this helps.

Thanks for the insight

How about Van Den Hul cartridges? Which version should I get for my JMW9 arm? (I am also planning on upgrading it to a 10.5 arm, unless anybody else could tell me otherwise.)

I am also looking at Zyx cartridges, which I think would be a better match for the JMW arms?

Toufu, you should also consider the Dynevector and Lyra lines of cartridges. Both work extremly well in the VPI arms.