The Sonata Plus 550, Solo (if you buy a quiet PSU) and Sonata Designer 500 are the quietest. I don't know about the P182 and P183, but the Sonata Plus 550 and the Sonata Designer 500 definitely are much quieter than the Sonata III.
Quite is relative. One person's quiet pc can be another person's jet fighter breaking the sound barrier.
I totally agree with you. My son says that a Sonata III is quiet and to me it is too loud (he's a gamer and I'm not). It also depends on the environment. In the family room, I wanted a PC that you couldn't hear until you were very close to it. The power LED indicates when it's powered up - I don't need to hear it. The Sonata Designer met that requirement.