I have not heard this before -- the 8 ohm speaker is easier to drive - speakers fluctuate in impedence. So even an 8 ohm speaker may have a minum to maximum range of 2ohms to 30 ohms but on average they stay around the 8 ohm range. Conversely some 4 ohm speakers may be relatively flat at 4ohms with a minumim of 3ohms and a max of 15.