Two other suggestions.
1) Instead of importing classes into UML disgrams, import the packages. (I don't know if the UML tools can import at so macroscopic a level.) 2600 hundred classes means maybe 20-30 packages (you can ignore the minutia subpackages at first). What does each package do? How does it relate to each other?
2) Use case main lines. What are the primary use cases of the system? When looking at these, don't bother with the exceptional cases (which I find often take up a good percentage of the use case), just look at what's supposed to happen. Explain use cases to your co-workers,and ask them to each describe the 5(?) most important use cases to you (see how much they all agree on what's the most important :-)
--Mark
hershey@vaultus.com