subwoofer volume doesn't increase

I have a 10" alpine type R 2ohm dvc powered with Alpine mrd-m605 at 4ohm. so it's getting 400wrms and sub rated at 300wrms. I'm using 4awg from battery to distr. block and 8awg from distr. block to amp as well as 8awg ground (manual says to use 4-8awg). ground wire is about 1 meter going under a 14mm bolt under my rear seat. sub is in a sealed tube enclosure estimated at a about 0.6cu/ft. HU is Alpine CDA-9857 (2V pre-out). The problem is when I turn up the volume I hear the speakers play more loud but the sub volume stays the same at some point (after about 60% volume) kinda like a flat spot. I can hear the speakers scream but the sub is not getting loud enough to keep up with the speaker volume. Need some expert advice please.

You need to set your subwoofer level adjustment control to SW1 (volume changes with system volume.)  Check your owner's manual I believe it is part of the BBE system.Support the12volt.com

Dyohn is right.   if you hit the button below the volume control this will pull up the main menu, scoll though that till you get to setup...then go you SUB SYS and select the mode that is not selected, not sure if its one of 22009 0-1000 Trunk WR 154.0DB 2009 1001+ Trunk WR

2007 USACI World Champion

2007 World Record

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that's the setting I have set

then try the other one, there are 2 settings...one adjusts with the music, the other is a fixed output2009 0-1000 Trunk WR 154.0DB 2009 1001+ Trunk WR

2007 USACI World Champion

2007 World Record

2006 USACI Finals 2nd Place

You said the sub flattens out at around 60% volume...you sure that the sub and amp are at their limits and can't get any louder??

tried the other setting and it sounds similar.