An amp signal input is 0.3V-3V. And a normal HU is 2.2V, some have 5V and some even 8V. By surfing www.bcae1.com, you can see the HU pre-out voltage and amp input voltage relations. There it will show you at what point the amp will 'clip' and so on. After you read it, you will feel that is high pre-out voltage HU good for you?

Rockford

Is 1V to 4V actually, don't know which manual I read it last time. The link above describe it all on the gain controls.

The best is turn the HU to max and use a multimeter to check.

Well, multimeter can test both sides maybe, but if cannot, if HU output +ve goes to MM -ve, maybe will not comes out reading. When doing this, try to have the manual on hand, atleast you can check which is which.
Details can go to
www.bcae1.com
for futher infos on ICE. A good start for newbie.