It is certainly not that simple. First of all transformer currents are RMS AC and not directly comparable to the DC currents after the rectifier. While it is clear that AC power must at least equal DC power, AC and DC voltage levels are typically different, so currents must be different as well.Do I have this all right?
It is not quite clear to me how manufacturers rate their power transformers and if all of them use the same rating method. I did some power calculations and it seems that current ratings are often based on some mean or time averaged amp power. Means that there are unknown assumptions about average power consumption/output involved.
Hammond specifies their 50W PT at 150mA@690V corresponding to a power of 103.5W and the 100W PT at 420mA@350V corresponding to 147W. While the power of the 50W PT corresponds to the calculated power demand at full output, the 100W PT value does not.
(From your post above it is not clear if you mean a 2x6L6 amp or a 4xEL34 (Marshall 1959) amp.)