I don't agree and don't disagree with buying headunit first but there are many ways to work around standard head-unit. Audison bit one, JL cleansweep or alpine processor.
Sure, sound quality will never be like from ODR, F1 or HXD2 but for a lot of people will be more than enough.
Problem is that by keeping standard cd player you will have to spend probably more money on additional equipment than on a decent head unit and it will still not be the same sound wise. Only one positive side of standard cd player is fitment in the car.
And yes, amp might give better sound quality even with standard system. Simply because speakers get more power - are controlled better.
Problem is that they will be running overtime all the time if you connect them to amp. Another thing is that not sure how you want to connect an amp to standard head unit which doesn't have RCA's.
If you're thinking about using adaptors high-> low - forget about sound quality.
I would start with standard sound system and sound deaden all doors properly (dynamat, stinger).
Doesn't matter how much your sound system is worth, it will never sound good without deadening.
You will be amazed how much more bass will show up after that.
When you do that, think about adding more components.
If you buy decent front component speakers you will need to run them with an amp.
Not even the most powerful cd player will be able to give enough power to decent comps.
Forget about all the 4x60W written on CD players. If you get 20 watts continuous you will be surprised.
Cheapest solution is to go for a 4 channel amp.
You can connect front speakers on 2 channels and later a sub bridged.
Disconnect rear speakers. Use only front and sub.
