What is the order of severity amongst the following?
- sound post crack on top
- sound post crack on back
- bass bar crack
anything other major things I am missing?
I sincerely hope you don't have all three, or even one, in that list!
Post cracks in the back. There seems to be a movement away from 50% hit on value for many older, fine instruments, but on a modern instrument it is still the worst. We have better repair techniques so the long term prospects for a good repair that causes fewer problems down the road are minmized, but if a living maker's instrument presented with a post crack in the back I would try to get the maker to make a new back.
Order of severity to value:
On a related note, how do you properly treat a violin to prevent the possible formation of these types of damage?
Don't drop your violin, don't send it through checked baggage on an airplane, don't ever leave your instrument in a closed but unlatched case (violin) or a case standing open (cello), and don't ever, ever do the youth orchestra thing of leaving your instrument on your chair during break. Those cracks happen as the result of some sort of trauma to the instrument.
Mary Ellen, sadly, it is not just youth orchestras who do that thing of leaving their instruments on chairs. I've seen it happen only too frequently in relatively good adult community orchestras where even experienced violinists do it - adults who should know better. Once, during a rehearsal break of one such orchestra I tried to count the instruments left not only on chairs but also on the floor, miscounted (so much for my math degree!) and decided it was quicker to count those with their violins safely in cases; there were just two - the CM (a retired pro) and me.
One way to properly treat a violin to prevent those cracks from forming is to use a modern suspension case. The design suspends the violin corpus from the end blocks so it can't take blows at the bridge or the middle of the back.
I'll add that it's important to avoid humidity extremes.