Problems setting gain

I just got an arc audio 2100cxl.  I checked it out and had to hook it up right away.  I was setting the gain and for some reason the gains will only start to change if the volume on my HU is at like 15.  The HU is an alpine cda 9831 it goes to 35.  I have the internal amp set to off right now.  I put the volume at like 75 percent then started to mess with the gains (it has a left and right gain above each rca) and nothing.  It stays at 61v until I turn it way down.  I need it at 38.98 (I think) and the only way that happens is if the vol is at like 15-17 and the gains are turned just about all the way down. I mean they look as if they ARE all the way down.  Any ideas, I'm stumped?  The volt/ohm meter was on AC in case you were wondering.

You're looking for about 400 watts at 4 ohm bridged, and reading the output at the bridged terminals on the amp? 

BTW, the following is something I just picked up with a Google search:

Mike

CXL-2100: "I would highly recomend this amp. i was previously using a mmats amp and also zapco ag350.the zapco has very similar specs to the 2100cxl.when i took out the zapco and put in the 2100 i noticed i had to turn the gains all the way down on the 2100 and i noticed improved sq from my focal components.if your looking for a 2 ch amp with killer sound quality this is a great choise. i am very happy with the performance and quality of this amp"

Maybe it is not at all unusual to have the gains like that.  I would guess that, because there are two gain controls, that each channel should be set to read 20 volts, then double-check at the bridged reading.  It still has to be done with the HU at its highest output setting.  If you are having to take the HU volume all the way down to 15 to get it set, then I guess 15 will be your max volume..  I only wish I had an Arc amp to experiment with, though...  ;)

I'll try the two channel approach, and yes around 400 @ 4ohms at the bridged terminals.  I've read some of the reviews as well but I missed the gain thing.  I listened to the sub after I set the gain with the volume way down and it would get too loud or quiet with each increment of volume.  I hope your idea works, I didn't like the way it jumped like that.

Check out ebay, you should be able to find some good deals. I saw one of these on there go for 165$.  Used, but if it works fine you'd get it cheap.  Thanks