Under these conditions, a resistor will almost certainly go open, not short... likewise for the rectifiers. And if they did go short, then the fuses come to the rescue anyway! This has not been a serious problem for the many tens of thousands of Fender Twins, at el, that are out there in every day use. With respect... I fear, you may be 'stiring the livestock' on this point. There is far more chance of a valve rectifier or other valve failing, than a SS rectifier diode rated at many times more than the current required for a simple retrosaurus-thermionic!