Speakers I already have, but no, they are not out of the question. They just offer a different view to the same research question.
I've been looking at AKG K 1000 long and hard. There are still some questions remaining, but yes, this looks like a decent model.
If I don't go for those, I will try to find the best closed circumaural, two wire (separate cables for each earpiece), ultrasonic capable phones that I can find (with some available calibration data).
Seems to be a tough one that latter one as well.
PS My preferred Halcyon reference is to thet track of the same name by Orbital, but diving gear is fine by me too