KT, I'll ask around, but I have a hunch that we don't have hardpan in the classic sense around here. Just soil of some type (combination of clay, silt, sand and some loam) over solid sandstone at some depth.
I neglected to mention that my goal isn't really to break up hardpan per se. I am trying to incorporate certain amendments, like lime and gypsum, more deeply in the subsoil in order to raise ph and correct some nutrient imbalances. For the large areas around the property, I have hired folks in the past to deep rip after the amendments have been spread. For example, last time, after we spread upwards of 20 tons of lime and gypsum per acre, I hired a guy with a big CAT (a D9??) to run a set of 3' rippers three different directions. If I can rip half that deep with my modest Tractor, I could do some good in the limited areas around the property that haven't been previously amended.
I have a tiller, but incorporating lime in a few inches of topsoil won't change the ph of the subsoil for a long, long time, if ever.