I had also experienced this exact problem for a long time until recently. It was really frustrating. I even took it to an authorized repair centre who said they had fixed it and said all it needed was a cleaning of the internal connectors which can become oxidized after a while. However I brought it home, fired it up and the noise was still there. The issue is something to do with the tubes and not necessarily 'bad' or 'microphonic' tubes. I replaced my tubes with a matched pair of EL84s and a new 12AX7 from JJ Electronics. This brand seems to be especially recommended for DT25 amps. I then biased the EL84s to 25-26mV. Problem solved. So it could simply be a case of biasing the tubes you already have. Or, it might be worth buying new tubes altogether AND biasing them. JJ Electronics highly recommended. I was so relieved to have finally solved the problem myself. One other thing to note is that you will probably only hear this crackling noise in Class A/B mode and not in Class A.