IMF's as a breed had pretty benign impedance loads of around 8 ohms right across the board. be warned that a transformer coupled valve amp will have next to no damping factor and in the IMF's case in particular, this may change the bass reproduction slightly - my Compacts don't like the Quad 303 for example when wide ranging percussive electronic stuff is played (oh alright, Depeche Mode from recent masterings ).
Good luck...
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