I think Larry did this for his pair.
I've been keeping up with this thread since page 10 or so, and I'm wondering, what are the cons of the 13's? All I've seen people talk about are the pros. There HAS to be something wrong with 'em... right?
In that sense it's just another custom with bass somewhere between those two. Granted, I've never heard either of the UE's, I'm just going from what Jerry has told me about his former creations. The low bass is pretty slammy but it's so tight I don't think the anti-bassheads will complain too much. Plus it doesn't bleed into the mids because the emphasis is only on the sub bass.
The frequency response is pretty much exactly what I would consider perfect for an IEM, but your mileage may vary.
Other than that, the difference is more under the hood - basically with the newer/faster/dual drivers. It seems to be nice evolutionary step in the technology.