b series use some weird size in the doors too, but you can get off the shelf adapters to accept 6 or 6.5" speakers, so theres heaps of choice.
google reviews on the size/ type speakers you want to get an idea of what ranks highly. dont just buy brand name because it wont mean they will automatically be good.
https://www.caraudionow.com/best-value-6-5-component-car-speakers/
split system speakers will have a better sound than coaxials, or even 3 ways. but a bit more work to install as you have to replace the tweeter as well as main speaker, plus theres a crossover box to fit in the door somewhere too.
in general the higher sensitivity a speaker has, the louder it will play with the wattage available. eg a 98db/w will play louder than a 93db/w on the same head unit. so take note when selecting speakers.
hope this helps