Hi Leroy,
Let me introduce you to Dr. Gary Wade
http://www.rifeenergymedicine.com/services.html
Gary has published some of the best papers out there.
The machine he offers uses physical vibrations to induce resonance with whatever the target is.
So in trying to move back in line with the Thread title, it will be found by reading his website that his unit runs a range of frequencies from 0hz-80Mhz. I believe that will most likely cover the frequency range of your concern.
Another analogy on Amplitude is the one spotlighting Ella Fritzgerald in that old memorex commercial.
In that example Ellas' voice set up a resonance with the wine glass and when her voice was Amplified the glass shattered.
But to your question regarding the amplitude of the devices on the market, it seems reasonable to me that the amplitude requirements of someone who carries 100lbs of body fat would be much higher than some one with a extremely low body mass index since lipids act as insulators to electrical current.
I hope you enjoy Gary's papers