Interesting data Plougot. Thanks for taking the time to gather that together. But isn't it exactly what VSL's 'natural volume' does, i.e. give you starting values for the relative volume of the orchestras instruments in proportion to each other? In any case such measurements are a very good starting point. It doesn't free you from knowing orchestration though. From your table we can gather that a bassoon has a dynamic range of 30 db. That means that it's loudest possible sound is 30 db louder that it's quietest. It can be very different for each register though. The low register will have a smaller dynamic range than that, (probably around 15 to 20 db) simply because a bassoon can not play as quiet there. The opposite goes for it's high register: the dynamic range may be very similar to the low register (15 to 20 db maybe), but this time because the high notes can not be as loud as in the deep register. With that in mind, you would have to set an individual dynamic range for each register of each instrument. I think that's not currently possible in the VI Pro Software. And while it may be nice to have, I think that it isn't terribly important either. If you use VSL's 'natural volume' feature (and Dietz has explained how to set it up properly) you should be fine in most cases. When making a mockup there are far more noticeable deviations from a real recording than the instruments balance. Interesting as this discussion is, I think it focuses on a tiny detail that most people won't even notice.
If you have Sibelius, Wallander's Note Performer is a good tip. It uses algorithms (derived from real performances) to shape the playback of your score to be similar to a real performance. It won't sound as realistic as a mockup, but it's a great tool to check such things as the balance between the instruments etc. I guess that may come somewhat close to what you are wishing for. And in my opinion it is better suited to 'teach you how it normally works' than a set of samples, which also has to cater to the ones who need the freedom for their daily work (there is a rather large community of people using samples for media compositions. It's actually much bigger than the ones who do 'classical' music with samples, I guess. And for these people 'reality' isn't the first priority, or even the second. But rather consistency throughout the instruments, flexibility, freedom...).
As an aside: I don't think Tchaikovsky made a mistake there, far from it. I think he noted an ideal that a performance should aim for, eventhough it can't be executed literally (a bassoon really can't play pppppp there). The problem in the recording lies with the clarinet, which is too quiet already so that the bassoonist has no chance for a smooth transition. He should help him out there and play a bit louder than that, while still keeping the overall impression of a morendo.