An analogy I just thought of with SH's comment: I work in the semiconductor industry, but that doesn't even matter. We have these high tech tools that do etching, deposition, rapid thermal processing, etc, of wafers. One of the "rules of thumb" you learn, is that equip is much more comfortable running in the 15% to 85% range of any particular setting (gas flow, power, pressure, etc.). Less than 10% or more than 90% is asking for trouble. I never even thought of that for power amps in a home theater!