I read somewhere that BCB iron can be run substantially above its ratings as long as the low frequencies are rolled off below 300Hz. The BC iron is rated down to 50Hz I believe? I cant find that article or post.
But why waste a good broadcast quality modulation transformer to run space-shuttle audio?
First of all, you will need a reactor, unless you have one of those rare BCB mod xfmrs designed to carry DC through the secondary. Then consider the original working voltage of the transformer, and compare that to the voltage you will be running. Some BCB transformers are tremendously conservatively rated. In my HF-300 rig, I use a mod xfmr taken from an RCA 250 watt broadcast transmitter. I have run the final at 1 KW DC input and hit 150% positive modulation peaks with no problem. The only rating listed on the nameplate of the transformer is 2500 volts DC maximum. That is exactly what I run to the modulator, and the final runs at 2 KV. Since voltage is what toasts transformer insulation, I think I am still running the transformer within ratings. Plus, I don't normally piss-beat the transformer to that degree. I have an identical spare transformer that has been sitting on the shelf for over 30 years without ever being called into service.
Back in the 60's I modulated a KW with a little Electro-engineering mod xfmr that came out of a 250w Gates transmitter with no problem. I added a spark gap across the primary and set the whole thing on insulation so that the core was floating.
Most of my mod xfmr blowing experience was when I was experimenting with high level speech clipping with a splatter filter between the mod xfmr and final. When I took out that crap, I never blew another mod xfmr.
I have never heard anything about suppressing the lows making it any less likely that a modulation transformer would crap out. I would take it with a grain of salt. Chopping off the lows at 300~ is kind of like slopbucket and Daylight Shifting Time; it is alleged to perform miracles and solve the world's economic and energy crises, reduce the crime rate and prevent the spread of AIDS.