These are about the same responses I got when asking about car audio. "EP600 in the trunk" and "M80s for the rears"
Actually, what you will find is that just about any "aftermarket" speaker will improve your sound as long as it is properly powered and getting a clean signal from the head unit.
I would say that there IS a noticable improvement, obviously, in upgrading the speakers, but many times, the best improvement (beyond that) is to replace the head unit and any factory amp that is in there.
You will get a lot of people chiming in about their favorite speaker brand, JL, Polk, Alpine, Kenwood, yadda, yadda, yadda... If you are running the basis stock speakers now, any of these will be an improvement.
What year/make/model is your SUV? Knowing what size speakers your vehicle can support will help as well for some specific experiences, plus you can always go to Crutchfield and put in your car information and it will telly ou what fits, you can get their product research on them, as well as customer reviews.
