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.
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.
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.