I have good earing and I really need to put my ear close to hear my Seasonic S12-II 520w Bronze.
I prefer having fans because when you don't you will start hearing the electrical noise (different on a tablet/smartphone/passive laptop where the components are compact with less "protruding capacitors and the like" which reduce electrical noise).
[quotemsg=19131122,0,1841500]It's been a long time since I read about the 600W fanless Prime model... THAT is the one I want.[/quotemsg]I have a 450 W fanless Seasonic Platinum PSU. It's orientation dependent and it'd probably be a good idea if your case had an intake fan. Especially for 600 W.
Even then, it might not get enough airflow if you have a graphics card that exhausts a lot of air out the back.