Bingo. You are correct.
Consider your example..100 wrms into 8 ohms, 40 volt peak, 5 amp peak.
20 feet #12 is 1.71 milliohms times 40, 68 milliohms. Toss in 10 milliohms more for the connections, you get 78 milli..
5 amperes in 78 milliohms, I 2R, 25 times .078, 1.95 watts dissipative loss in the wire. Does anybody consider 1.95% to be below audibility? (this is just a calc of level of effect)
Now, lets add the tweet signal...100 watts..for a monowire, the peak current will be 10 amperes, the peak loss is 4 times previous, or 7.8 watts, out of 200 watts..
The peak loss using one wire is twice that of biwiring, yet the rms loss is the same..
Keep in mind, 20 feet of #12 is rather robust...I use #12 for 100 foot runs, as they meet my need..