Special thanks to everyone who participated in this thread. I could not have gotten here without you.
The primary purpose of this thread was to see if there was any correlation between the sound of a Grado headphone and the length (or depth) of the cups. Based on some time comparing various Grado headphones using the superb FR graph tool at head-room I cant say that there is actually much correlation. A longer or deeper cup does not produce more bass reliably, nor does a shorter one or anything else. A few headphones with similar lengths show vastly different FR graphs, and a couple with different lengths show some clear similarities.
Part of my goal in seeing if this correlation existed was to prove or disprove for myself at the very least whether Grado uses the same drivers in all of their headphones which has been a popular rumor for some time. I am comfortable saying that Grado does not use the same drivers across their line.
A secondary objective for this was to select a new length for a friend to turn a set of cups for me to mount a driver in. I have nothing too exciting to report on that front. I think Im going to go with a 34-37mm deep enclosure like the vast majority of grados already have and put SR-60 drivers in them.
Something interesting I did find which as far as I know has not been posted yet are some interesting construction details on the HP-1000 family of headphones. In order to measure the driver mounting dimensions of the various grado drivers I had to take apart my HP-2. No biggie, they just unscrew and come apart. Its quite easy really. Anyways that I noticed is that where my RS-1 (which I also took apart for other reasons as described elsewhere) have a fairly smooth internal surface the HP-2 have a very unusual step compared to any other grado. The diameter of the chamber is actually larger than the diameter of the driver, neat! I also noticed that the HP-1000 enclosures are made of 2 pieces of aluminum which are joined at the top where the grills are attached, rather than 1 piece of wood or aluminum as in the John headphones.