Based on a review I've read, one difference is the amount of gain each of these driver tubes provides. The Ray Samuels Emmeline II Stealth preamp will accept any of these tubes without any other adjustment, thereby altering the amount of gain produced. The review is at http://www.6moons.com/audioreviews/emmeline2/stealth.html, and the reviewer tried two of these three tubes with the result being more or less gain (12AX7 has the highest amount).
