Using two drive blocks is definitely the way to go. What I've found is the fx8 really does differ depending on setup, I've changed the speakers in my combo 3 times and each time I've changed the combo of drive blocks that I've used. This just means you have find what works for your rig. The shred distortion has the most gain on tap but out of the box it sounds thin and a bit shrill but when used with another block you can get some very meaty tones out of it. I've used the shred with the ruckus before and that was good but found the bottom end a bit compressed for higher gain stuff. Try the shred with the acb, this is currently my favourite drive combo.
Another good tip is to try changing the hi and low cuts, this can significantly alter your tone. I tend to drop the low cut on most of the drives to get a more open sound.
Last but not least try lowering the sample rate, I've found this really works on drives, maybe it's my ears but I find this makes the drives sound more natural and can also lower the very high end.
Have fun!! I've found the modulation and delays the easiest to get a good base sound but the drives take a bit longer to dial in.