Which type of lathe do you have?
Its an old Scully (well, *all* Scully lathes are old...) equipped with a Westerex 3D cutterhead. The cutterhead does not need a lot of power and has a moderate impedance, we are easily able to drive it with our M-60 amplifiers.
Kiddman, it helps to take a look at what is happening in the groove with a microscope. Of course every lathe has one, you can't do the work without it! We have a multi-color LED light source that we installed, so we get different colors depending on what we are seeing- the lands, the groove wall, or the bottom of the groove. As I said before, the defining quality of groove depth is the center of the groove.
If you have high frequency modulation, that shows up under the 'scope like a groove with modulation on the walls. If you have bass information, the *entire* groove moves from side to side with the bass modulation. If the bass is out-of-phase, the groove will stay in one spot and seem to vary in width and depth. This sort of condition will knock the stylus right out of the groove and so has to be avoided. In natural 2-mic recordings it does not show up- only if the recording is multi-tracked to start with.
I Googled for images of this stuff. Without a multi-color light source it can be a little hard to see, but there is a photo that appears at this link that is not bad for what I am talking about http://en.wikipedia.org/wiki/Gramophone_record
Scroll down aways, the photo appears on the right hand side about half way down, just above the heading 'vinyl quality'. If you look closely at the grooves, you will see that they squiggle a bit. Those larger squiggles are bass notes. This is why I say the groove depth does not vary all that much, as the *entire* groove has to move to make the in-phase bass note.
