No head unit is going to power the speakers properly because they're nowhere near the power of even a small dedicated amp. Here's why - Amplifier manufacturers do not need to follow specific guidelines for power rating tests, they can test and rate it however they want.
Head unit power ratings are usually based on some sort of maximum output with the easiest load which is 1-ch driven at 1kHz until it hits 5% or more distortion which is so bad it sounds like crap. So when a head unit is rated 200w, what that means is 4x that 1-ch max rating whether the radio can realistically do it or not. But to hear music, a radio or amp needs to reproduce all of the sound from 20-20,000Hz, and that output requires 3-5x the power to push all that sound out. In order to do that, physics plays a role - output circuits need stable power and current from a well regulated transformer and capacitor supply which costs a few bucks to make. Cheap IC circuits don't have much in the way of power supply regulation so they run out of sustained power quickly and distort. In reality, a decent head unit can push about 5 watts of clean full range power to all four speakers before it starts to run out of current and voltage peaks clipping the audio waveform which is heard as distortion. The woofer cones of speakers need several watts of power with good AC voltage waveforms just to get them to move some air for bass output, which is why adding a 25w or bigger decent amp makes such a big improvement to the sound output. Not just volume, but clean full range (as close as the speaker can manage) sound that you can hear properly. Good speakers are also important because if they can't do anything properly with the signal, they will sound dull and flat like the cheap paper cone speakers in a low end car.
A good amp will have proper ratings which would look something like this:
50w/ch x 4 at 4 ohms from 20-20,00Hz with less than 1% distortion.
A head unit under the same requirements would be rated about 5w/ch.
Good amps will be rated with the specs above so you can compare them. Cheap amps just claim wild numbers they can't hope to achieve but it sounds good to unsuspecting customers. The really good stuff will be rated well, but perform above their ratings, but they cost more because they're well made with quality components that perform above minimum spec.
So to answer your question
@dbfl look for an amp rated about 50w/ch in the $200+ price range and you should probably be ok for basic needs. Ignore similarly priced amps claiming 100w/ch because they're usually IC driven junk that runs out of steam when playing sustained music and will likely fail when pushed hard overheating an IC and blowing the amp = wasted money (but if you take it easy and don't push it, it'll do ok). Another way to know if it's a decent amp, is to look at the fuse rating - 1amp for every 10w of power is a rough guide. So a 400w amp will require a 40amp fuse...if it has a 10 or 15 then it's over-rated and can't sustain the power ratings claims.
