preamp voltage and gain range

i have an Alpine CDA-9857 HU that has 2v pre-amp voltage, my amp is a kicker sx1250.1 with 2 12" CVX12-4 wired to 1-ohm. when i am setting up my gain range from within the amp, i have choices of 1,2,4,8,16.   i set   it to 2v seemed a little quiet, so i was changing settings around and tried the 1v setting and it was much louder with no audible distortion (75% HU volume 60hz tone @ 0db).   what problems if any could happen by setting my amp's gain range to 1v when the HU pre-amp is 2v?

It is rated at 2volts...you should have the sub level on the head unit all the way up...volume up to max before distortion. And use a DMM and this calculator to see what your output voltage should be for the power you want at whatever load Voltage calculator