Just replaced my HU with an Alpine 9884R. This is supposed to have 4v pre outs, same as my old unit, however the sub seems very quiet compared to when I had the old unit in. also the sub seems to be louder at lower volume than it does when its fully turned up. I'm due a battery replacement soon so wondering if this due to the amp not receiving enough power. I was under the impression that due to the preouts being the same voltage I could leave the gains on my amp at the same level as they were previously or am I wrong?