Setting my gain

Ok    I did the search on settin gain, went through JL's tutorial, but I'm still not sure. I have a Xtant 121m mono running it at 250W at 2 ohms. According to E=(P*R)^.5 the gain should be at 22.36? This is extremely low on the dial, like around 1/4, and I have to really turn up the bass to hear it at this setting. Can't be right can it?

Alpine CDA 7893

Xtant 121m

IDQ10D.4 V.2

You're not wrong.  The input gain is designed to match the sensitivity of the amp to the voltage output of whatever is driving it (like the head unit.)  The way to set it using the voltage method is, basically, to set the head unit to its max non-clipping level and adjust the input gain on the amp until you are measuring the the proper voltage output for the max RMS rating into whatever your speaker load might be.  In the example above, 250 watts delivered into 2 ohms requires an output of 22.36 volts.

I understand the formula...the voltage would be the square root of power in watts X resistance in ohms...but what does the .5 in the formula above represent?

The power output form an amp is

P=I^2*R   Were I, the current pushed through the sub is, I = E/R substituting E in the first equ. gives

P = (E/R)^2*R  So then P = (E^2 )/R  and,

E = (P*R)^.5 or the square root of the desiered power output ( not always the maximum printed on the amp) times the impedence of load.

All terms above are RMS vales

For my setup the speaker had a DC resistance of 3.1 Ohm, but an AC impeadence of 6 Ohm, so my 400W 4 channel amp is making the 40V it should but only producing 266W of power. 

Scratch that ...  its a 4 Ohm 2 channel.