[Herald1360]

The resistance of the windings on a transformer is just that, resistance. It's not a particularly useful guide to number of turns, turns ratio or whatever since it depends on the gauge of the wire used as well as turns number and size of core etc etc.

The number of turns on an output transformer primary will depend on several factors, not the least of which is the compromise between cost and low frequency performance made by the designer. Needless to say, the better the performance required the (much) higher the cost.

As long as the turns ratio is somewhere near, and the primary dc current ratings are similar between the two transformers, the substitute will work OK. It may sound a bit different but in a mono unit that's probably not much of an issue as long as it sounds "OK".

You don't say which Black Box you have- the Fenman 2 amp is push pull but not ultralinear connection. If your BB is UL, you'll have to arrange screen feeds for the output pair and the sound quality may suffer a bit.