<< Changed my rear 80mm to a Panaflow Low 80mm. Made nearly no noise difference. I mean it's quieter but not enough to justify cost. The two front ones on selection speed 1 is super loud and noticeable. If I were to change fans, I'd change those two. Funny thing is they're all the same fans but the front two seem much louder. >>
After much playing around with the case. I found my case to be completely reverse of your situation. When the two front ones are set to low, It's barely noticeable while the one in the back sounds like a blowdryer.
Gonna start looking for a blue 80mm enermax to replace the rear.