Robert
Tapehead
From my own experience (limited) and those of more knowledgeable members of this forum, 12" doesn't make for 'lively' compared to 9". The extra length bestows other virtues. Again, maybe it's the incompatibility of the 3012 with your cart. causing an imbalance of presentation.
I had a 3012 (maybe very different to your incarnation, however) in 1969/70, but they were supposed to go with m/m cart's then (Shures, mostly), and wouldn't have suited a modern low compliance m/c cart., I would have thought.
Maybe an SME Five-twelve would do the trick; it would certainly float my boat !
This was also my understanding until recently but I have since seen the light, due in no small part from speaking with Tony.
The older 3009 arms were medium mass with the supplied shells - medium for the 9" and medium-high for the 12". You could increase this with heavier shell.
Those arms should be fine with MC.
The later Series 2 Improved versions reduced the mass and were designed for the higher compliance MMs of the 1970s.
I'll be trying a MC in a very early Series 2 myself soon.