being loud will come down to a few things such as what you run the speakers off how much power (wrms not just watts) and the sensitivity.
eg head unit will be about 16wrms depending on brand etc etc. if you run them off an amp that is 100wrms the amp will not distort at the same level as the head unit.
sensitivity has a part as well if your speakers have a sensitivity: 89dB/Wm a speaker that has a sensitivity 91db/Wm needs less power to produce the same sound as that with 89db/Wm.
i hope this helps mr 32
Always use an amp on at least the front speakers and the subs.'ALWAYS'