sound quality not good at infuse, which codec?

Hi, 

need youre help guys, 

At home i have the following setup;

jailbroken ATV2 with infuse
Airport extreme with external hard disk
and a Bose Cinemate GS 2.1 set at my tv

the thing is, when i download a movie, there is a difference in sound quality, for example, bass is harder or softer, in general the music is not balanced well. 

it sure has to do something with codecs or something, but can you tell me where i need to look at when i download a movie? 

( normally i download a package of 5 gigs and 720 or 1080p to be sure that the quality is good.)

thanks a lot in advance, 

 

 

Mark