Another interesting one is phase noise and distortion and crossover distortion (if you use a simple class-B amp) means you can send a sine wave into a high power amp, but the amplified noise level at audible range might be icky. As a personal anecdote I've noticed this with commercial ultrasonic cleaners. Sure... noise level might be 30 dB down at audible range, but pump a 100 watts at 40 KHz into it and I'm listening to 0.1 watts of noise at audible freqs which is quite loud indeed. Theoretically ultrasonic cleaners should be inaudible, but high powered ones are not. Much as intermod distortion is hilarious with high power RF, noisy/corroded/semi-faulty ultrasound could be loud as heck acoustically thru no fault of the generator, amp, or transducer.