You will need to get a new HU... back in the 80's they used to sell EQ/Boosters that would work with your stock. I'd try new speakers first and see how that sounds. Probabaly need an amp later...
You don't HAVE to upgrade your HU. It is a good idea but not a necessity.
You can either get and amp that has the capability of high/speaker level inputs or you can buy the adapters similar to these:
JL Audio XB-CLRAIC2-SW - RCA to Speaker Wire
or these:
Pie EIS-LOC-AFv2 55W Adjustable 2-Channel Line Output Converter
If you want to maintain your Fade and Balance capabilities, then you will have to have 2 of these adapters. One hooked to the front outputs and one to the rear outputs from your stock stereo.
I would recommend just replacing the speakers first and seeing if that doesn't get you the clarity that you were looking for. If that does not do it, it is a toss up between a nice amp or a nice head unit. It depends on what you are really after. Do you want clarity? Do you want volume? Or, do you want clarity and volume? After replacing the stock speakers, both a head unit and an amp will increase both the volume and clarity capabilities. However, I feel that if you are truly looking for more clarity just replacing the head unit will suffice. If you are just looking for volume, an amp with a stock stereo will suffice. If you are looking for both volume and clarity, you will need to replace the head unit and add an amp.
Just a note of how all of this works. Radios are generally purchased by consumers who look only at how many watts it is capable of putting out. Amps are the same way. But what people don't realize with head units is that the HU is only putting out the max number of watts when it is turned all the way up.(literally) Also, consumers generally don't understand the RMS rating. The RMS rating is the number of watts that the HU is constantly putting out. The number that is advertised, such as 45x4, is the peak output. Now, keep in mind that most HU will start to distort at about 75% of Maximum output. So, when looking for a HU, pay attention to the RMS rating in order to ensure that you are getting a quality unit that will suit your needs. The same thing applies to the ratings on amps, but their RMS is much closer to their peak.(With quality units)
Speakers have a similar rating. You need to pay attention to the amount of watts that the speaker is capable of handling as well as how many watts is needed. There should be both a peak, in which you don't want to exceed, as well as an RMS value that you want to use as the minimum.
For example, these speakers:
Boston Acoustics S35 - 3-1/2" Speakers - Sonic Electronix
They have a peak rating of 120watts per pair or 60watts each. And an RMS rating of 60watts per pair and 30watts each. If I determined that these speakers were the ones that I wanted, I would make sure that the HU or amp that would be powering them was capable of at least 30watts per channel but did not exceed 60watts per channel.
You can see from the specs on these speakers:
MB Quart NAN216 (NAN 216) 6" 2-Way Component Car Speakers System
That they are capable of handling more power but also require more power to run efficiently.
*note: I am not endorsing one speaker over another, It is just the first speaker that came up in a google search.
I know that it is a lot of information to process, but it is just like any other system whether it be a sound system, a computer system, or a lift system for your truck. If you go out buying bits and pieces hoping to slap something together, it will probably end up being something that you regret. Piecing something together is okay, as long as you have a goal in mind and stick with it.
