I believe that the original D26 series tweeter was 4 ohm... Try to verify that... If it is, put a 2 ohm resistor in series (positive lead) with the tweeter. This will do 2 things: help to bring the crossover frequency back to its original design and it will pad the tweeter a hair to help take care of the brightness that you speak of... again, I am going by memory of that tweeter being 4 ohm, check it out, if it is this inexpensive part will help,
Tim