OTOH how much should one spend for an amp? What are the threshold of measurements at which amps differences are inaudible?
Depends on the speaker. If you have something that doesn't have bizarre impedance swings and power requirements (i.e., the majority of speakers), maybe a couple hundred bucks will get you something good enough for blamelessness. If you have electrostatics or speakers with incompetently designed crossovers (cough, Wilson, cough), plan on adding another zero at the end of the price tag.
As for thresholds, that's where things can get complicated, but if the output impedance is low enough to not cause frequency response deviations of more than 0.1dB, you're likely more than safe. The days of engineered amps having distortion high enough to be audible are many decades past.
Note, "engineered" rather than "designed." Pick the latter and all bets are off.