The effective mass of Scoutmaster JMW-9 tonearm?

I found out from VPI that it's 8.7 grams.

You are about right I imagine. If you search Vinyl Asylum, some would say 7 gm. some say 9 gm. In any case I find that high compliance cart. are the best match. I find that ortofon carts. are very good with my JMW 9. I just got an Ortofon SL 16 MKII with a 0.015mv output. It sounds better than my Shelter 901 with the JMW 9 IMHO.