How will changing the concentration of a Tris buffer affect amylase enzyme activity?
I realize I'm 4 years late, but wanted to give an answer to this. It appears that tris often has inhibitory effects on amylase activity. Adding more Tris will lead to competitive inhibition with the substrate and will skew the results.

Binding of Tris to Bacillus licheniformis alpha-amylase can affect its starch hydrolysis activity
https://www.ncbi.nlm.nih.gov/pubmed/18289113