Hi, to answer your question, the 6 ohm winding is to be able to handle all loads well with a single tap. When multiple taps/windings are added, you increase inter-winding capacitance and decrease bandwidth and fidelity. I found that a 6 ohm winding meets all loads best. For a load > 6 ohms, it is absolutely no problem. For a 4 ohm load, it is still compatible. It is just a coincidence that Lou's speakers are 6 ohm.
I will say however, that they are especially magical with Lou's speakers!