As mentioned, I've already tried the "secondaries disconnected" test. Loud buzzing from transformer for 3-4 seconds and then the fuse goes. I've now spent nearly as much on fuses during testing as a new transformer costs...

I went for 55+55 as this was the closest to the existing 58V without going over. That's been running a set of three M752 drivers for around three years. While the theoretical offload voltage may be 1.4x that, I'm not sure what the actual voltage is with the three drivers permanently connected, even with motor current zero. I'll measure it and report back once the new transformer is installed, for those curious about such things (as I am - I probably measured terminal voltages when I first set the thing up but I can't remember that far back). Looking at the rectifier/capacitor board, the smoothing caps appear to be rated at 68V, which might also be a bit marginal...