(d) It may and upon request of either the assessor or the objector shall compel the attendance of witnesses for hearing, except objectors who may testify by telephone, and the production of all books, inventories, appraisals, documents and other data which may throw light upon the value of property, and, with regard to an objection that is subject to sub. (7) (c) or (16) (c), may, on a showing of