You can improve the low freq response of any xformer by lowering the
souce and load Z. The low freq response is determined by the
relationship between the xformer inducance and the source and load Z.
It is not magic.

I have a cheap xformer that was designed for a telphone application.
It is rated at 600 Ohms and when driven by 600 Ohms, it rolls off below
300 Hz which is fine for a telephone applicaiton. I am unsing it in a
wide range audio application and I drive it from 8 Ohms. When driven
from 8 Ohms, the same xformer is flat to below 50 Hz.

The low freq distortion is a strong function of the dirve level and the
size and type of material of the core.

Have fun.

Mark