I was thinking of putting some better speakers in the doors of my 2003 1.9D van. Are there any speakers that just replace the existing ones without any modifications?
I'm an anti-audiophile, but an options tart - sort of having a foot in both camps!
The original speakers are 165mm (6½ inch); a common size - look at Fleabay for pointers. Your main variable is whether you seek better sound quality at the same level of amplification, or if you want a better quality sound at a louder volume. This latter aim can lead you towards replacing the head unit, and adding a power amp or two... (I am aware you said "without any modifications".)
Bear in mind you can also enhance the listening environment with products like "Dynamat" inside your doors.