T_bone, your first example is what I was talking about, not the latter. 100W on a 1500W transformer is no worries.
The formula for calculating the distortion at the output of the transformer is E = I x Z (if this seems similar to Ohm's law to you it should).
E is the voltage of the distortion component
I is the current being drawn from the transformer
Z is the impedance of the winding of the transformer.
Bigger transformers with greater wattage have a lower Z. So you can draw more current before the distortion voltage becomes significant.
By the time you exceed 50% of the capacity of the transformer, the distortion component will have risen to an unacceptable level. It is the 5th harmonic (300 Hz in the case of a 60Hz fundamental like we have here in the US) that is the most deleterious to electronics. It will make power transformers and power rectifiers noisy, making for degraded sound quality.