[Arran]

Levi;
I think that the 6X5 has really been made the scapegoat for other faults. In the case of your old set I think it probably was a shorted filter caps, or possibly the tone correction cap on the output transformer primary, the latter takes out more power transformers then one might realize. The reason I say this is that it doesn't appear that the tube was misused or overstressed in the circuit, the 6X5 was paired up with 6K6s in lots of other radios, especially car radios, without incident, and your radio only had six tubes in the entire set. I think that the maximum current load for a 6X5 is 80 ma, varying with plate voltage, so basically they are for the AC equivalent of a five or six tube AC/DC set.
I actually haven't run into a set equipped with a 6X5 that had a failed power transformer, but then again I don't run into many brand Z radios where I am. I have however run into more then one Canadian RCA or G.E set (same chassis really) with a bad transformer, but those used a 5Y4, they just happened to use power transformers that were barely adequate for the job in the 1940-45 model years, but I've never run into a Philco or a Rogers set with a failed transformer, though one guy I knew claimed to have a Rogers with a failed one.
Regards
'Arran