i've just tried it with a TM018 http://www.mouser.com/catalog/specsheets/XC-600134.pdf
10k/10k xicon transformer.
inductance of the primary(same as secondary) is 3H( on my LCmeter)between external leads(10k) and 0.75H between center tap and an external lead(2k5, these impedance are average things like in any transformer).
so i made choke switchable between 3H/0.75H with the tone push/push pot and find that a value in between will be great something like...let say 1,5H!
Modman your xicon transformer have a 20K winding so it is 5K between center tap and one external lead,may be not so far from the 4K value listed 1.5H
when using primary let secondary unconnected(floating)
Edit: this link may be helpful http://www.lautsprechershop.de/english/index_tools.htm
go to RLC calculator