Bobby at Merlin wisely led me into Cardas tri-eutectic. Replacing WBT with unleaded Cardas improved everything in crossover-- better detail and bass delineation with less hyped up sound. Perhaps the benefit of unleaded solder is greatest in high-mass joints as found in xover wiring. However to flow well it needs a hot iron with a thick tip that maintains heat, and even then joints look ragged.
For delicate jobs such as surface mount I've been using Radio Shack 4% silver-- its main advantage is availability in fine gauges, and it seems to sound OK.