[Arran]

They must have had a reason for adding transformers other then to eliminate the hot chassis problem, switching to a floating B- would have eliminated most of the problem, and all it takes is some times, and some wire, so they needed the set isolated for some other reason, maybe to connect it to other equipment. Usually when I see things like this my temptation is to put it back to stock as 9 times out of 10 it doesn't work right, if at all, you must have got lucky in this case. What I further don't understand is why not leave the series string tubes and just use the isolation transformer for the B+? Some factory built sets were like this, Rogers and Electrohome made a few models this way, in fact I think that the tube heaters were operating from the transformer secondary in those sets, but I will have to check.
Regards
Arran