Pin down will help keep the ball rolly. The low differential on the Rumble will keep that piece from flaring as much as the HyRoad Solid, which has much more diff. Their RG's are similar and covers, I would put the Storm cover as more angular and the Roto cover smoother. Do you want same motions but different strengths? Or one more mid lane read roll and the other cleaner and more angular? I hear a lot of different "compliment" stories in the shop, and the customer have a different idea than myself.
I put my pin in my ring, and cg 1 inch right of center grip on my Rumble. It ends up being about a 4x4 with a double thumb weight hole. This ball is angular for me with the hole. Before the hole, it rolled the whole way down the lane and sometimes left flat 10's because it was too rolly. The DT hole really added about 3-4 boars downlane. I went up on the surface to 4000 as well. My HyRoad Solid is a 5x4 pin up with a hole 5 inches over from center grip. For me, they roll similar but the HyRoad Solid picks up better on fresh conditions due to core numbers.
If you want a similar roll compliment, go pin above the ring on the Rumble and pin above the bridge on the HyRoad Solid with cg placements being the same. So a 4x4 or so the Rumble and a 5x4 on the HR S. Now all of this is based on my stats. 450 rpm 18 mph. 5x1 up PAP low tilt medium rotation. I do hope some of this helps. If you have any questions, feel free to ask.
