The hearing on the proposed assessment shall be held by the trustees of the district sitting as a board of adjustment and equalization at the time and place specified in the notice provided for by section 39-1623. Such date of hearing shall not be less than twenty days nor more than thirty days after the first publication of the notice thereof, unless such session be adjourned with provisions for proper notice of such adjournment. At such meeting the proposed assessment shall be adjusted and equalized with reference to benefits resulting from the improvement and shall not exceed such benefits. If the owners of all properties on which special assessments are proposed to be assessed file a consent in writing with the clerk of the district to the proposed assessment, the notice and meeting for adjustment, as provided in this section and section 39-1623, need not be given or had.