I always thought that the Amount field would show the amount in the Subsidiary currency that created the invoice but I am not seeing that behavior. What determines what currency the amount field is showing?

If you're looking at a transaction, there will be a currency field on the accounting tab. Whatever currency is there, that will be the amount on the transaction. Subsidiary comes into play when you're looking at searches, reports, and Related records like revenue arrangements.

For this case I am referring to a saved search report.

Amount is always in the subsidiary currency unless you're showing consolidated exchange rates (in the edit search screen, go to the results tab then check the consolidated exchange rate field) I guess maybe multi-book would act differently too, but I've never used multi-book