how hard does the amp work?

If a amp is say 400x4 @4ohms and 800x4 @2ohms and you have  4 subwoofers that are 100 watts rms. Would the amp work as hard if you had the subwoofer hooked up at 4 ohm each to get out 100 watts. or to connect 4 2ohm subwoofers so the gain would only have to go half as far?I can't hear you!

The power rating on a speaker means nothing in terms of the load it will place on an amp.  The speaker's impedance is what determines how much current it will draw.  A 2-ohm load will draw approximately twice the current of a 4-ohm load at the same voltage level (Ohms' Lam.)Support the12volt.com

You shouldn't adjust the gain if you change the ohm level (or the output of the amp) as the gain is NOT a "volume" control - it IS a compenstation adjustment for the input of the amp to match the output of the head unit you are using.  Once the gain is set you shouldn't ever have to adjust it, no matter what sub combination you throw at the amp.Kevin Pierson

So if I have the amplifier adjusted to exactly to 100watts for a 4 ohm subwoofer (measured on meter and all). and I put a 2 ohm subwoofer on it. it would automatically get 200 watts rms? and if so is the amplifier working any harder or is it working less. or the same?I can't hear you!

Yes, if an amplifier produces 100 watts into 4-ohms and you connect 2-ohms to it (assuming it can handle the 2-ohm load) it will produce 200 watts with the exact same settings and input level.  Producing more power of course requires more "work," so to speak, as well as more power supplied to the amplifier from the electrical system.  Nothing is free.Support the12volt.com

Thats what I was lookin for Thanks alot for the help guys.I can't hear you!