I'd been wondering about the Hagerman, and found a used FryBaby3 on the cheap, so I decided to give it a whirl. I'm more curious about what the circuit actually does, than how it affects the sound of cables. But as long as I have it, I may as well try to assess both.
First, I was curious what kind of bandwidth the device had. Roughly, I'm seeing full amplitude signaling between 0 Hz and 200 kHz, with a bunch of high frequency content in the noise. Also note the nature of the waveforms. Fairly random, no square wave or saw tooth waves, etc. By contrast, the Audio Dharma FAQ says it uses a square wave sweep. I'm not sure the square wave offers much advantage. I see MHz range harmonics from the FryBaby3 already, and the amplitudes of this content is again so low as to likely be irrelevant.
Second, I was curious to see what kind of output it had. The manual says 1 Vrms. I'm seeing what looks to be closer to 1.4 Vrms in LINE mode. The spec also says the output current limit is 10 mA. These strike me as being very low for a "cable cooker". Contrast them with the Audio Dharma specs of 120 mA and 12 Vrms. Comparatively, then, the FryBaby3 is somewhat of a toy. Or at least, I expect no acceleration of "burn in" using the FryBaby3 vs just using my DAC. In fact, many DACs will have an output that is hotter than the FryBaby3's. Of course, the brickwall filters in most DACs will kill most content much above 20-30 kHz. Maybe that's an advantage for the FryBaby3.
At 50ms/div and 1V/div below, we can see the FryBaby's amplitude sweep pattern. Average voltage, visually, looks well below +/- 1V. Maybe around +/- 0.7V.
Now I'll plug in a new phono cable for a few days, and see if I can detect any audible difference. I'm pretty skeptical that I will.