Byron, welcome. No, in a typical situation listening to music a 0.7% THD shouldn't be noticeable. In some tests using pure test tones heard through headphones a THD as low as 0.3% was detectable, but with real music a threshold of 1.0% or even slightly higher was found.
Yes, THD is a function of power output and remains very low up to the rated output number and then generally increases quite rapidly. Incidentally, despite the term being quite commonly used for some reason, you're not "pushing" power into a speaker; it takes only what it needs for a given output level at a particular moment. For speakers of typical sensitivity playing at a comfortably loud average level that's about 1 watt. Brief split-second peaks use much more, of course.
You're paying unnecessary attention to both THD and speaker impedance. As was indicated above, distortion in the well- designed receivers available these days is a non-factor at least up to(and usually beyond)the rated power output. Speakers vary in their impedance at different frequencies and a "6 ohm" speaker may average that over a certain range, but attempts to finely evaluate distortion possibilities aren't very realistic.
