76doublebass:Given the very low output of this SET amp, SET's typically limited bandwidth, and the higher output impedance you typically get with tube amps, I'd say that lowering the speaker cable impedance to near zero would yield dramatic results. You did notice that the big jump in sound quality came when you added the fourth run of cable.
To my ears on my 1.8 watt 84C ZEN SET Amp this cable just sounds absolutely incredible,But may sound very different on high bandwidth solid state.
On a more powerful system where the amp has a very low output impedance, you might hear less difference lowering cable resistance than--say-reducing skin effect or inductance.