Remember that there is no such thing as "half way up"
Gain is a function of the output from your headunit. Set the inputs on the amp to absolute 0 (counter-clockwise , or all the way down) put in your "Bass that Ate the Earth" music of your choice. Turn the head unit up about 3/4ths of the way. If it reads volume with numbers and lets say it goes to 35, then turn it to 30. Now go and turn up the sub amps input until the sub actually sounds like it is clipping. Not the "clipping" light as that is just a input voltage sensor.
Also assuming that the amplifier can actually make 2400 watts the current draw will Be somewhere around 280 amps! So you ,of course , wired this car with 1/0 gauge wire (power AND ground),and beefed up your battery ground as well as adding an extra battery...RIGHT???
Don't add a cap as that won't help under-current issues. Caps are GREAT! Just don't do what most people think they do //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif