I'm using 12-gauge zip cord (because I had a reel of it) and it's undetectable by feet placed between an area rug and wall-to-wall carpet in my listening room.
And the rug/carpet doesn't do anything to hurt in the floor-reflectance department.
More seriously, I used 16-gauge twisted-pair Belden cable to wire an Allen Computer Organ to it's speakers, as specified by the manufacturer. The organ amplifier is at the front of a church, and the speakers are at the back. The cable runs are about 175 feet. If there is any loss of high frequencies as a result of that, I'm sure the worshippers are happier because of it. I used the same stuff (I had bought a 500' reel) to wire a pair of JBL CBA-50 speakers to their amplifier in runs about 40 feet long. Given that the room already reverberates low frequencies abundantly, I really only wanted to emphasize frequencies in the upper speech range. But I have played music over that system and it sound excellent, even with the bass rolloff from those small speakers. I'm quite sure any high-frequency roll-off would be slight at worst.
But the twisted pair of the wire does help to reduce noise inductance.
Rick "uses $10 line-level interconnects also" Denney