The easiest way to a/b would probably be to construct an IC with a bunch of solder joints patching the signal wire together. I have no idea how many solder joints it would take to become audible though...
As far as ease of use, saying lead free solder is worthless for the hobbyist isn't the conclusion I came to. The Johnson solder works great with the iron at 800 F and makes nice looking joints. I built my entire amp with it and had no issues using it. I am experienced enough to know what a cold joint is too...
The heck with a bunch of solder joints, just use the solder instead of the wire in your ICs. That way you'll be able to assess if there's any discernable difference with 10 + inches of solder from one plug to the other. Use identical lengths and set down for an extended listening test.
Psst: It's already been tried, but the results were "very" subjective, IIRC.
BTW: I said that lead-free solder is
"nearly" worthless for the hobbyist, not that it can't be done. You are the proof that it can, but why anyone would, given a choice is beyond me. Manufacturers who must comply with Government regulations is one thing, but unless there isn't a choice or it's all you have, I certainly wouldn't bother as there are TTBOMK no advantage to leadfree solder.
Best Regards,
TerryO