Hi Mark
The offset pot adds either plus or minus current into one of the inputs of the op amp that forms the front end. That current originates from the plus and minus power supplies within the amp. Unless perfectly clean (which it is not), it will be a noise source input into the amp and the amp will amplify it with the same gain as any other signal.
I am sampling the BC550B so I don't have any comment on these yet Mark until I receive the samples and verify them. On paper, they look a little better than the TIS97, all around. I would not infer anything from the pricing. The MPSA18 appears to be the best device sampled so far. There are more contenders out there as well. With the TIS97 unobtainium at this point, I had to look for viable alternatives.