Please keep in mind that this is your decks problem, not the volt. The problem ultimately lies with the fact that most phones and mp3 players do not have a line out. I had the Creative labs zen:vision and the connection dongle had a line out which was perfect for connecting to decks. Unfortunately no so much with my Ipod touch and phones.
The issue is most line in's, especially on car decks, run the signal through a resister if the line is amplified. This can really hamper volume levels, especially if you have a shiat deck with few controls. Currently however I am using my LG volt as my music player through my sony deck. I dont remember the model, but it is about 4 years old with a line in on the face. It was about $130 at best buy.
My rule of thumb for running with amplified line in is;
#1 turn the players EQ off. Now I am going to turn around and say I ran my ipod using the Loudness setting, but you really don't want to mess with the audio spectrum on the source side, let the deck handle it. It is bad enough that you are running the sound through 2 amps, 2 equalizers will just screw up your sound.
#2 no way to get around it, but you will need to run the devices volume at %80-90. I don't suggest maxing it in case the phones amp fails and breaks up when the amp is maxed out at %100. Also the less you amp the phone, the more you will have to amp the deck and you will blow your ears (and speakers) when pulling the cord out.
#3 check for a line-in level on the deck. Mine allows me to increase the volume of the line in even more, this increases volume before being equalized. Opposite of the main amp.
#4 manage your bass and treble levels on the deck only.
If you don't feel you are getting enough bass, it's your deck and your speakers. Adding bass from your phone just muddies the sound. I find I get alot of my thump if the main deck volume is high enough. At low volumes I lose a lot of bass.
