Re: Mark vi sharp low c
Do you have an alto or a tenor? Some altos have pitch issues in the lower register because of the bottom bow configuration. The long bow varieties in particular play sharp on C and below. The long bow horns started around 80,xxx, but this is only an alto thing.
That being said, if the pitch is changing for you at different volume levels then it sounds like its probably a user error over an equipment error. I would recomend playing long tones into a tune at all volume levels for 15 minutes every day. If you do this consistently and concentrate on keeping the pitch even and the tone unwavering, I guarentee that your sound and pitch will be better after just 2 weeks.
Nice snag on the VI by the way, sounds like you got a good deal. You should also have the sax looked at by a local repair tech with vintage saxophone experience to make sure its in top playing condition.