When I began to upgrade the 2006 L322 Range Rover Stereo with an aftermarket amp, I initially went with an off the shelf cross over system. Later I built a set of speakers for my home office (post coming), and learned how to build crossovers.
Building crossovers is actually really simple for single order, which provides a gentle slope. If you go for first order crossovers it is important to crossover tweeters about an octave lower than you would with a more aggressive crossover.
Wait! Octaves, First Order! What the fuck am I talking about here?! Ok, let’s back it up a bit. An octave is basically a doubling of frequency. Most speaker systems aim to provide a flat response between 20hz-20khz. Most tweeters crossover at 5khz. So if you are running a first order crossover, you should aim to build your crossover at 2.5khz.
Once you know the performance of each speaker, you can design custom crossovers. To find the parts needed for each web based crossover calculators are useful. I used this one https://www.diyaudioandvideo.com/Calculator/SpeakerCrossover/
You input the impedance, of the woofer (low pass the signal), and the tweeter (high pass the signal), for two ways such as the rear door speakers, or three way for the front doors. I choose to utilize a single order crossover made out of really nice components I sourced from ebay and https://www.parts-express.com.
Different component types can color the sound, and it can be useful to overbuild these components with nice ones that can handle high voltage as necessary. On ebay I was able to find capacitors and inductors from nice amplifiers which were very nice.
This is just a general outline of course. Three way crossovers, and even resistors paired to tweeters can help. Upgrading crossover components can also help improve a car stereo’s performance at high-volume by ensuring the speakers get clean signal they can play. This strategy was very effective in stereos that you want to keep some of the stock electronics but they don’t sound very good at high-volumes.
In general I like to replace all of the transducers and crossovers. I think doing that in many instances can improve the sound greatly without gutting the amplifier and processing sections. This can be advantageous because HK, Meridian, amplifiers in Land Rovers and Jaguars are very advanced. So it can be improved while keeping the functionality that may be inbuilt into the amplifier section.