How does the meaning differ for the following two sentences?
- Even then, the subject seemed as fascinating a problem as there could be.
- Even then, the subject seemed as fascinating as a problem could be.
The first one assumes that there exists a set of finite fascinating problem. The subject, then, is a member of that set of fascinating problem, and is fascinating itself.
The second one does not assume that there is such a set. IT says, however, that the subject is fascinating to the extent of how the problems can be fascinating.
There's no real difference.
If you put the sentences under a microscope, you could analyze them like this:
That is, the second sentence doesn't literally say that the subject seemed to be a problem. But the reader is unlikely to come away with any other impression, so it doesn't really make a difference.
(Unless the context strongly implies otherwise, but we have no context here.)