I've owned their top of the line XLR3. Warm and organic. But maybe a bit shy on details and speed..but I only noticed it compared to the Siltech Classic Anniversary series IC. I did prefer HiDiamond over Nordost Frey and Heimdall.
HD 9 RCA should be very similar to XLR3 as I understand. I've always thought HiDiamond's have very good price to performance ratio. Good weight and never fatiguing.