Right from the classic audio site:
Q: What does the 'B' designation on some components mean?
A: Not much, really. Sometimes, it means cosmetic changes. Sometimes, circuit changes. Sometimes both. Nothing you can depend on, though. At the most, it means, "We're going to do this better, somehow."
There are cases where the model completely changed in the transition from non-B to B designation (like the 2285 and the 2285B; sometimes, there is little obvious about it at all, like the 1200 and the 1200b integrated amplifier.
You can also find models that have electronic changes internally only (the 10 and the 10b are good examples of this... after making about 100 model 10s, they reworked the IF strip on the 10b, for one thing), and models that have new features and just the required cosmetic changes to accommodate the new features, such as the 3250 and the 3250b, which adds a moving coil amp for the phono input.