Why only odd harmonics after signal is amplified? *Plots inside*
The even harmonics, like 2f , 4f , are synchronous (to) the zero crossing of each half-cycle of alternating-polarity and thus (with) perfect 50% duty cycle (it) has no even harmonics, as they cancel out.

Whereas 49% or 51% duty per cycle adds even harmonics that are 1% of the input or approx -20dB in power density. ( not exactly as I left out some details). 


  With this wisdom you can measure a square wave symmetry on a spectrum analyzer for slicing a sine wave with offset far easier than using a DSO then make a conversion table or null adjust even harmonics.