mrmb,
The only impedance curve I could observe for Soundlab was the A1 on Stereophile. I will attempt to insert it here. If this speaker was used with these Fidelium cables there must a change, I would consider large, in the frequency response, more so with the 10 foot version. That change, based on the graph below which may not be appropriate, would mainly be a gradual reduction in high frequencies starting at approximately 2KHz, increasing to a 3db drop by 10KHz, and 6db at 20Khz. My ears have not heard 20KHz in many decades, but I would notice a change at 10KHz of several db (at least I think I would). There could be a small emphasis close to 200Hz that is audible.
Perhaps what this cable does is lay bare the audiophile claim that they "really know there systems and would know if something changed". I don't mean that specifically to you mrmb. Unless a speaker has constant impedance, this cable must make significant frequency response changes. I don't think that can be debated, though the amount of change will depend on speaker impedance. Some users really enjoy this cable, but I have not read any comments (other than negative) that indicate a realization of significant changes in frequency response.