I am hesitant to tell people to do something other than what is in the literature, unless there is a good reason. Obviously if there were other literature on this enzyme in different buffers, that would be helpful. Borek would know more, but I don't that that ammonium acetate has much buffer capacity at this pH. Other buffers that had more buffering capacity might be used at lower concentration. If the enzyme prefers relatively high ionic strength, one might add KCl, to compensate for the hypothetically lower ionic strength of different buffer.
Regarding the pH shift, my previous comment was strictly about pH changes during dilution, not about any changes in pH due to the sample itself. If I am following your proposal below, 25 µL of a 1 M stock is the same as 50 µL of a 0.5 M stock.