a little in depth explaination please.

1.  A gain is a level-setting device.  It matches an amplifiers output to the voltage of the input.  Read through this forum SEARCH on the subject of setting gains.  There is no such thing as percentage in determining gain setting.  You are assuming that full to the right is okay to do when you use a percentage to describe where you turned the screw.  Depending on the voltage inputted from the head unit, the amp's gain screw may be set anywhere along it's rotation.  Normally, the gain will be properly set more towards the beginning of the rotation...or more to the left side.  Here is a good in-depth explanation of gains - www.bcae1.com

2.  A two-channel amp was devised to produce stereo sound.  It consists of two amplifiers, one labelled "left channel" and the other labelled "right channel".  The two mono amps (channels) are packaged into one unit and is called an amplifier, or a 2-channel amp.  Each channel is fed a signal, one being left stereo, the other being right stereo, and the amplifier amplifies those signals, then outputs to the speakers.

3.  Two channel amps are ideal for components because of the need for stereo sound.  It will provide power for a set of components or two individual components.  A four channel amp consists of 2 two-channel amps combined and will provide for either two sets of components or four individual components.  Mono (one channel) amps are ideal for the mono output of subwoofers.

4.  Passive crossovers consist of a combination of inductors, capacitors and resistors....chosen and wired together in such a way as to provide limits to the frequency range or relative decibel level any individual speaker is allowed to play. 

Crossovers in your home audio speakers may look something like these:

The tweeter and midrange of a component set are separated by a crossover.  More info here:  https://www.the12volt.com/caraudio/cross.asp

In fact, take some time to read through all the help listed in the left column of this page, as well as bookmarking the link I showed you for many days and nights of reading and education.  The search results for gain setting should adequately answer your questions in that regard.

Cheers!

Shouldn't this be "mono?" It's not stereo until the two channels are used in tandem. Might be some confusion there...