Good idea, a sub woofer. But for that you can even use a special transformer which is optimised for that.Regarding the high frequency rolloff of transformers such as the 1628SE . . .
Most likely cause: a Very high leakage reactance between the primary and secondary (poor high frequency coupling).
Next level factor: High distributed capacitance across the primary.
Lowest level factor: The absence of Litz wire.
At the other end of the spectra:
The 1628SE had the best low frequency rolloff of any single ended transformer I ever bench tested using the
$50,000 Rhode & Schwarz Vector Network Analyzer.
Driving the 5k primary with a 1k impedance, It was not yet rolled off by 1dB at 10Hz.
(or to say it this way: not yet rolled off by 3dB at 5 Hz). Sub 5 Hz response indeed!
If you have a pair, you might build subwoofer tube amplifiers out of them.
Just my $50,000 opinion
In post 36 you see a 3300 Ohm transformer with about 4,5Hz to 100kHz bandwidth. Can be easely lower if you wanted with a little bit loss at higher frequencies.
At least you can do something good with the Hammond The Fiat is nowhere good