Hi ...this is my first post

I begin my story...
I had an amplifier for my composite speaker (a 4 way) ... (sorry for my english and i dont know a lot about car audio specification... ) but i had disturb sound that was coming from my engine's car. So the salesman change my amplifier for another one (supposely the same quality!) ...
But im not sure... i think my sound is a little worst than before :1

Anyway ...I ask for a better amplifier... he asked me 150$ more for it.

My old amplifier was :
Soundstream 4 way channel LW4.480 ( i bought it around 350$)
The new amplifiers is :
Soundstream 4 way channel EGA 560.4 (ha ask me 500$ for)

He told me that he got this new amplifier for a cheep price (normally its around 700$)

I want to know if you think its a good deal or not?... if i should keep my old amplifier??

thanks in advance (and ill do my search too :p )