Whenever you remove the power supply from your audio system to replace RCA cable,amps,speakers,etc it's very important to be aware that you shouldn't not plug the RCA cable to the amps prior to turn on the system.
These is the following steps
After the installation is done have all the amps level adjusters to the minimun level, i mean all of them,same apply to the head unit.
Place the key on ACC & turn on the head unit,after a few seconds check the amps led to make sure they are on & running.
Recheck all the ground cables for any loose nut,this is very important.
Turn off the head unit;Plug one of the subs input RCA cables,then turn on the head unit & rise the volume little by little,if possible have someone with you to adjust the amp levels at the time you controling the head unit.
Repeat this step until you've complete connected all the input cables to the amp.
When you had all the RCAs connected run the head unit for a while,then beging to adjust the EQ levels on the head unit's volume no more than 60 % of his power,after you had this one sorted go to the amps,with the volume at 60 % or less adjust the levels until you begin to hear distorsion.
If you amps come with high/low level inputs have them positioned accordingly
especially if you running the speakers & subs on the same amp.personally i don't recommend that,i rather run one amp for the speakers & another amp for the subs,don;t forget to add an capacitor 1.5 farads or higher.
Why i don't recommend running all the speakers & subs on the same amp /
well,the amp has more tendency to over heat & if it fails & go go bye bye you'll run off of music completly
Another tip to have in consideration is the amps,subs & speakers impendancy.
For example :
If you decide to go running with 1000 watt subs at 4 ohms look for a amp at the same impendance spitting no more than 950 watt,the RMS is also very important,if the amp's RMS is 500 watt please make sure the sub holds on that amount or more RMS,more likely for precaution & peace of mind
If speakers is what we talking about i rather run new wiring instead of touching the factory wiring to run the new speakers,yes is more job tought,but it pays its self.
I suggest that because when we decide to upgrade the audio sys the factory wiring isn't the one that fullly fits the new speakers specs & that takes away performance from your audio sys.
Dynamat the vehicule is a key for better & clear sound,and because the VX's interior is acoustic it's highly recommended to have this one done,that helps to hear the music quite high without turning the volume quite high
When adjusting the sub & speakers use the balance & fader levels to better adjust the sound,since the left side speakers are closer to the driver than the right side ones balancing the sound can definetly helps to get the best sound by just slightly adjusting this one over the left;The fader control level can be use to have the rear speakers spitting more than the front ones because the cargo/rear passenger area has more dimensions that the front/driver-passenger so you can have the music coming better from the rear & even from to right to left