6x9 would be fine for rear fill if you aren't too serious into this ice thingy. And no, there's no such thing as 6x9 being too noisy or performs lesser than component speakers, not most of it anyway. If you think it's too noisy then that's coz you got the rear fill sound upfront too much. Just try keep it at the back for your back passengers, there's no surround setup with 4 speakers. Some crappy components sounds even worse than 6x9, so I don't agree with that statement. If you have lesser space to give, try considering a pair of 8" 3 way speaker hooked up to maxxbass 103. Believe me, it'll sound a whole lot better than trying to cramp a single 10" subwoofer in the wrong enclosure.