XRAY - Model racing cars

Last weekend it was testing my new 2014 on low-medium grip carpet with Ride rex 34 outdoor tires (control tire). I was driving with the stock 2.5 springs and my car was pretty loose and hard to drive. (modified)

I tried 2.5-2.8 springs in the front and I didn't feel much improvement. Maybe it was even worse.
The I also tried the 2.5-2.8 in the rear, so same sprigns front/rear and this was a huge improvement. This car was a lot more stable entering the turn and it pushed a little on power, which was easier for me to push the car on the low grip surface.

So now I'm wondering the follows things:
- When do you use progressive springs in the front? And what's the effect of them? I've seen a lot of setups that use them.
- When do you use progressive springs in the rear? And what's the effect of them?
- When do you use them in the front and in the rear?

I've also read that on CPX tires, progressive springs on both ends are good. Why is that?

I wanted to do some more testing last weekend, but the track was closing pretty early.

We sometimes use progressive springs at the front of the car to have achieve a more aggressive feel, and more turn-in.

Using them at the rear, will further improve the turn-in, but the car will have less rotation from mid to exit of the corner.