Pro-ject Debut III Isolation

I recently purchased the Speed Box II as an addition to my Pro-ject Debut III TT. It currently sits on a 1 inch thick slab of granite on top of my component cabinet. How can I improve isolation ? Thanks in advance.
You might put something like the Isonodes under then slab of granite, to decouple it from whatever it's sitting on.

Pro-ject's Ground IT Deluxe works great under my RM10. If you can spend $400, it's a good value and it really works. Still, since you've already got the granite, I'd say decoupling it would be a good place to focus your effort.

I have a Debut II and have had success using the Bright Star Isonodes under the stock feet (which are surprisingly terrible). They're kind of a pain to set up with the soft feet on the Debut II (I recommend using a bubble level) but they work well and are dirt cheap.