Caveat, these are just "Old Wives Tails" of audio, they are not concrete rules, "Just Rule(s) of Thumb": (1) double the power of the loudspeakers manufacturerer's minimum suggested power recommendation, (2) for high impedance speakers use tubes, for low impedance speakers use solid state.
How much is too much...
Basically I have one question and one question only. Give me one rule of thumb when buying a Power Amp. What are the does and does nots :) of buying a power amp?