Re the Indigo cable and the solid/stranded question, while a case could be made for referring to it as solid core, IMO (based on what I perceive to be the most common usages of the terms) cables employing any form of Litz construction (i.e., a single overall conductor consisting of a group of individually insulated conductors) should not be referred to as either solid core or stranded. It's simply a different animal than what both terms are generally used to refer to.
As for the OP feeding his speakers with only one solid core #18 ga wire for each + and - terminal I would not think that would work too well for the sound of the speakers or for the amp. I will leave the technical why nots to Almarg and Kijanki.
I agree, as indicated in my earlier post, unless perhaps the cables are unusually short. Simply put, the resistances of typical lengths of 18 gauge speaker cable are high enough to conceivably/perhaps/maybe under some circumstances result in sonic effects that are at least slightly audible.
First, the combined resistance of the two conductors in a 10 foot run of 18 gauge solid core wire is about 0.128 ohms. For the OP's nominally 4 ohm speaker impedance, that would limit the damping factor seen at the speaker terminals to no more than 31, no matter how high the amplifier's damping factor may be.
Now a case could be made that damping factors above a few tens of ohms are overkill with most speakers, and that happens to be my opinion FWIW. But I don't consider my opinion to be the last word on the subject, and a lot of audiophiles and designers believe otherwise.
Second, when cable resistance is high enough to constitute a non-negligible fraction of speaker impedance, the interaction of that resistance with the speaker's impedance vs. frequency characteristics may conceivably affect tonal characteristics. 0.128 ohms is 3.2% of 4 ohms. Is 3.2% a "non-negligible fraction"? I don't know. But it seems high enough to suggest that it would be best to not take any chances, given that the percentage can be easily reduced.
On the other hand, I am aware that over the years there has been some advocacy for narrow gauge solid core speaker wire, even 20 or 22 gauge. I believe some of the writers at the British publication HiFi News & Record Review advocated for that twenty or more years ago. While I don't doubt that in SOME systems the results of that approach may be subjectively preferable, I would have to think that what is happening in most of those cases is that the cable is altering the sound in a way that is complementary to the colorations of the rest of the system. In other words, IMO it amounts in most cases to applying a band-aid.
Happy New Year to all!
-- Al
