I have a DP6 with 12" Red wand as well, but I don't know what the effective mass of it is. The mfr literature seems to be referring only to the 9" wand. However, my dealer says that the 12" Red is very flexible and has worked fine with every cartridge he's tried on it. My experience has been with a Benz L2 and Glider M2, Denon 103R and a Grado. All of these worked fine. I also heard a Koetsu Black today at the dealer using this setup, and it sounded excellent.
Dave
Dave