[Bastiaan]

for the increase of overall volume you can do several things:

- use audiosuite to normalize your tracks

- use the trim-plugin

- send a hotter signal to ptle when recording (this is the best!!!!)

- adjust (when recording) the software gain (in the hardware-menu i think)

As for monitors: these are very important and you should get professional ones. Dont play cheap on them. I use Tannoy reveals and i am pleased with them.
You said you are using a sony soundsystem that normally is used for gaming? Well.....those ones have some sort of eq built in to make everything boomy and punchy. Get rid of it and connect a normal amp with normal speakers (pref. proffesional monitors) and go from there. Mixing with headphones is a nono by the way....

Bastiaan