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Welcome to the forum! They are Lundahl LL9202 transformers.
All sine wave measurements posted are of the transformer setup. Of course, the BHSE can do 1500Vpp at 10Hz with a beautiful looking sine wave. Getting ~1400Vpp @10Hz from a transformer setup with a clean looking waveform is really good though; a little higher in Hz and it has more bass output than a BHSE (as the 20Hz 2kV measurement shows, was also getting a clean waveform at 13Hz@2kV). Up to you if you need the extra volume and additional bass output or not. A lot of people really like the LL1630's. As aokman shows in his video below, they are a little lacking in the low end, but otherwise have an outstandingly flat resposne without any additional components.
