Thanks for everyone's replies and suggestions and I'll try to address most of them in regard to where things stand. The two primary aspects of this that have me the most flummoxed are that 1) everything was up and running initially, but with a degree of bias and thermal drift that I tried to address by reverting back to the original semiconductors on the driver boards since the rebuilt unit performed perfectly with the stock driver boards from my working unit. It was when I re-installed these reverted boards that the pulsing DBT behavior began and it wouldn't power up properly. 2) If the output transformers, zoebels and meter driver connections are all correct, why does the unit power up fine when the fuses are pulled to disconnect them all from the output? I did check that the transformers are properly in phase, and since utilizing the 2ohm taps for speakers essentially bypasses the transformer it doesn't seem that disconnecting the transformers really differs from that situation. What would cause a unit to "not tolerate the DBT?" Will do. They are all new, plate type current sense resistors, and all tested fine when I re-pulled the heat sink assembly after things went to hell... Zoebels are good. Once I get the transformers wired back in I will once again dive back into the driver boards. Will do. And while I utilized new terminals and terminal housings (which were of course previously butchered) I left the the original headers. And while upgrading them may be a good choice, I don't know that I suspect them since the unit was operational initially. It was only after taking the driver boards back to the original semiconductors (which were all checked on the curve tracer before going back in) that this weird pulsing failure to power up reared its ugly head. Thanks agin for all the input, and I will report any progress.