3-way/2-way VS. single: A single sends the sound through the same cone as the bass which as you can hear from the stock speakers when the bass hits they tend to distort the mids and high frequency's. A 2-way adds a second mid/high speaker which combats the distortion but still the highs have a dependency on the mid not distorting to keep highs from sounding off to the trained ear. A 3-way adds a separate tweeter for just high frequency sounds, like symbols and electronic sounds as an example, and stops and high distortion from any mid distortion. This system works very well for automobiles due to not needing all the sound to be thrown all in one direction where a single excels, like for mic testing.
Any 2/3-way will be a major improvement over stock, no matter if you have an after market radio or stock with or without an amp to power them. FYI the stock units have a 25W speaker mated to them so if running a stock head unit an amp may be required to keep the stock look but most head units are now pushing 50W per channel so without an amp stay slightly above this. I personally went with 80W RMS speakers with a 50W per channel radio and am hard pressed to get them to distort because of over powering, but do distort because all radios distort sound because of being amplified over 66%-75% volume.
I snagged a set of these:
https://www.pioneerelectronics.com/PUSA/Car/Speakers/A-Series/TS-A6880F
My review with a Sony Xplod head unit rated at 52x4 Watts is they are not bad but could use a slight boost in power to reach peak performance but over all sound much better than the stock speakers. They could use a bit better mid speaker quality but at around $50 USD they are a great bang for the buck. I did a 4 speaker swap and can say that, with this head deck, they are quieter but sound quality trumps the perceived loudness deficit which is rectified with 4 channel low wattage amp.
My system, half out of commission ATM due to frying my 1000W amp, only really lacks a slight bit of mids but keep in mind that not all people can hear the differences that I can due to playing music on a guitar I have a trained ear for the sound. If you are an audiophile with an ear for proper sound these you should stay away from but if you really can not tell that much of a difference in half a fret of sound then these will soot the standard music lover.