What happened to you when you replaced the selenium rectifier with a diode and found an improper bias range is a common occurance. Because a diode has more "forward voltage" than a selenium rectifier, those two 10K resistors on the 7 lug terminal strip have to be made smaller to get the bias into the proper range. You are also wise to bias the tubes a little lower than Dynaco recommended. Biasing at the "1.56 VDC" that Dynaco recommended back then is now considered a little on the high side. With a stock driver board, a 1.40 to 1.45 VDC bias point will get you more life out of your output tubes and your amp will sound just the same.