actually it doesnt ruin the soundstage.... you just gotta know what your doing...
Depends what your version of ruining the soundstage is really? Do you think that when the rear speakers tend to over power the fronts is ruining the soundstage? or maybe when it feels like the sound is being dragged to the rear? or somet else?
Coz if you have a decent head unit & decent speakers/crossovers you can set it up perfectly for your driving position so it wont ruin the soundstage in the slightest. So to say rear fill ruins the soundstage is in a word "b****cks"
& its his choice if he wants it.... he asked a question... he wasnt asking to justify himself he was asking what was better for the rear.
& an answer to your question would be mate... I have 6*9s.... & I used to be hater of them myself, but after a little work, I made my own enclosures for the 6*9s, so not only did they not mess with the subwoofers air space, but the cones cant get damaged either.
& set up the delay between each speaker perfect for my driving position & the way I have it usually is mostly from the front but a bit from the rear so you get that surround sound effect going on!
You can do the same with all kindsa speakers in the shelf if you make enclosures for them! coaxials & componants included. Its up to you mate... if you have two sets of componants, it can sound too trebly! I would say if you really want rears... then go for coaxials, 6*9s or a set of dedicated mids (but they can & usually do cost alot!!)
But to get decent 6*9s that quite loud & good SQ cost alot! ive got pg tantrums & they are around £150 online! So if you wanted coaxials, I would say look at JL TR 6.5 & PG Tantrum 6.5" coaxials for around the 100 note mark!
hope that helps mate, any further questions, feel free to ask.