I have been listening to my new SME30-12 for the last week and can give some feedback. I have used the 'standard' 30-9 for many years, with both SME Five and Graham Phantom arms so have a useful point of reference. I won't prevaricate. The 30-12 is a surprisingly large improvement on even a late model 30-9. It has a noticable 'silkier' and 'smoother' sound; it loses the last vestiges of a vaguely 'mechanical;' sound that can linger with the other SME decks. (The Graham helps hughly to eradicate this on the 30-9). The bass on the new deck seems even deeper and more 'round.' The whole soundscape becomes even bigger and is also more relaxed. Noise levels are reduced still further. Frankly, it is an utterly thrilling deck.
Now, the arm....the SME Five (9inch) on the 30-12 still preserves much of the difference in sound quality over the 30-9. The sense of a smoother, plusher. sound remains. The 12 inch arm is l smoother still, a 'silkier' sound....... but at the cost of a slight loss of dynamics. Some might prefer the 9 inch arm. Either way, fine as the 30-9 is, the 30-12 is a real step up, regardles of which arm is used..
I believe that as well as all the extra mass, and the adjusted suspension cables, there has also been some minor changes to the internal structure of the hydraulic dash pots. Whatever has been done, the result is well worthwhile......very expensive I know, but still fine value among the 'super' decks. The combination of astonishing build quality and wonderful sound quality, plus great usability, will take some beating. I will have to economise for a couple of years to pay back the cost of the 30-12, but it feels a sacrifice worth making.