The answer is still the same. Front speaker is most important along with the head unit.
I think the most important is adjusting the fader, bass and treble is the most important of all before upgrading. I recently drove Acura RSX loaner car... sound was terrible at factory 0 level and someone's attempt to improve it with treble at the highest... When I adjusted the whole thing to the right balance and tonal quality... I was very impressed with the factory system.
First thing to do on any factory head unit/system is adjust the fader to more front... like +3 or +4. Front speakers are front stage where we want to listen from... not the back... back is just a surround 'minor' sound fill.
Then you raise the volume to a moderately loud system and listen... does it sound too boomy? Most systems are... so set the bass down like -2 or so... Then listen to the treble... can you hear it? Is the higher frequency clear and crisp? No. most system are not... so turn it up... like +2 or +3.
The above tuning instruction is for factory amp/head unit... even with upgraded head unit and factory amp... still applies.