People can and have done very scientific measurements of cable properties (i.e. ex-McIntosh engineers, someone that I would trust in the field) and measured sound output from speakers using various cables, but skeptics won't buy that.
When it comes to aspects of sound that are not yet measurable... The first step is controlled testing to make sure the aspect actually exists. The second step is defining what the difference is, and the third is determining the cause. It isn't good enough to just throw your hands up and say, "we can't know everything." You have to go through the process of trying to figure it out.