Please first upgrade your PRE... this will be the biggest difference in SQ...
Other upgrade have less impact...
According to my experience, the effect on SQ due to different upgrades are:
Player (Headunit, PRE): 35%
RAC cable: 7.5%
Amplifier: 20%
Speaker Wiring: 7.5%
Speakers: 30%
And this list is for branded stuff! If you use cheap stuff... its effect will be negative!
Also the effect of a bad PRE(Headunit) multiplies if you install an amplifier!
The factory headunit of Cultus is very bad (I think some Chinese headunits have better SQ than this!)... so again I would recommend a Headunit upgrade!