About decks you are correct on the preout voltages. On average single din decks run about 3 or 4 and double dins are lower. You can find higher preout voltages on double dins but I've mostly seen it on costly higher end decks.
I only mentioned 6x9 speakers because some prefer the easier install. I personally dislike 6x9's and 2 or 3 way round speakers. Im all about components. Some people are intimidated by the extra installation components require.