It's all down to trial and error to find the right transformer to use, and the right variable resistor for that transformer to work the way you want.  Start with putting the low impedance side in series with your speaker.  Then see how it sounds with the secondary open and shorted.  When you find the right transformer it will limit the high frequencies coming out of the speaker when open, but not excessively so.  Then try different variable resistors till you find one that gives you the range you want.

    I went to a Vocational School for Electronics in High School (like 55 years ago!)  At the end of the 2nd year, the teachers took us aside one by one and asked us questions on how we like the course, suggestions to improve it etc.  One question was if we've done anything unusual or unique in electronics; so I explained the Transformer used as a Variable Choke to them and they went silent and seemed to be deep in thought since it followed all the theory they gave us for chokes and transformers, but they had never heard of one being used in that way.  I'm sure at least one of them must have gone home and tried it.