Is there any way, through the use of external components, to extend
the range at which an audio output transformer rolls off high
frequencies?
For example, when using an 8 ohm to 100V PA transformer for voltage
step-up, amplitude falls off at around 25KHz.
The low frequency performance of an audio transformer is maintained
by using a mass of high-permeability iron. To get higher frequency
performance, one uses thinner iron plates with insulating spacers
(just oxide layer on the plates, really) to prevent internal eddy
currents, or for very high frequency one can use powdered-iron
or ceramic (ferrite) cores.
But a ferrite that performs well into the RF range is more expensive
(and larger) than laminated-iron for a given power level. And this
is an OUTPUT transformer, power level is high. If you can
use biamplification (split off the woofer or subwoofer to a
separate output circuit) the low frequencies can be handled
with iron core, and high frequencies with whatever is appropriate.
If it all has to go through a single core, you might need to get
a custom-designed unit, or use something meant for switchmode
power supplies (and the specs won't include '8 ohm').