Thus far, Wainwright's article is the most revealing discussion I have yet found. Also of interest has been a thesis I had downloaded some time ago by G. P. Jones - "The Significance of Water in the Pyramid Texts".

It would appear that the earliest text to shed any real light on the issue is Chapter 149 of the BOD - as Wainwright notes. This originates with the Papyrus of Nu, variously dated to either the first or second half of the 18th Dynasty. Gunter Lapp, in his publication of this papyrus, appears to lean towards the earlier dating (pp. 17-18).

What I do not yet understand is how or why the word for "cavern" seems to have switched from the "Tpht" of the Pyramid and Coffin Texts to "qrrt" in the New Kingdom period. And they do seem to have gone a bit "cavern" crazed during this era - perhaps something in the water, or in their "incense"?

Anyway, this is about as far as I've been able to get. Many thanks for your suggestions.

Lee