Ah, well, since I posted I've been reading up about transmission line technology, and it would appear to be a very different kettle of fish.
A transmission line affects the rearward output of a driver, with the intention of eliminating it's effect on the forward output of the driver, or, more commonly partially eliminating the rearward product, and bringing the remaining audible rearward product (bass frequencies) into phase with the forward facing bass frequencies to reinforce that part of the spectrum.
The SE846 uses the folded path between the forward production of the driver (BTW why do professional refer to iem drivers as "receivers"? that confuses me!) to limit the audible part of the output to chosen frequencies, IE bass.
I have no idea what happens to the rearward production of a balanced armature, or indeed if it even has any.
At least that's what I think it all means.
Please feel free to put me right if I've got it all woefully wrong.