THD on amps?
 

I am looking at amps and many say "< 1% THD" even higher end ones. Then others say 0.06% or some extremely low number.

One example is the Diamond Audio MICRO2V2 at 0.5% for $419 MSRP, and then there is the diamond audio HX400.2 with a THD of 0.06% for $569

Are they measuring differently or is the THD really that much lower? It seems like you would definitely be able to hear the difference.