I compared the two for several days before handing my original 13s off to a friend for reshelling. Here's what I found:
1. The Roxannes are largely brighter. That's not to say they're bright. But the 13s seemed almost impervious to sibilance. I think the Roxannes give it to me when it's in the recording. I did hear some hi hat type noises that were louder on the 13s. That surprised me, and suggested that not all of the treble range is more pronounced on the Roxannes.
2. The Roxannes' bass reproduction is clearly superior. When set to neutral, I haven't yet noticed a frequency bump. When comparing it to the LCD-2 bass, I found performance to be equivalent. That's quite an achievement.
3. Relative to the 13s, there's an added sense of impact. Perhaps this is a freqphase enhancement? There's a softness to the 13s. I don't know how else to describe it. The Roxannes don't have it though. They sound relatively sharp and focused.
4. Front and back layering is the most significant improvement for me personally. The Roxannes separate "in front" and "behind" sounds the 13s couldn't.
5. As expected, imaging is improved. The soundstage is surprisingly coherent.
6. The previous five points add up to a significantly more detailed set of IEMs.
Before the 13s came out in 2009, I had UE10s. The performance gap between those two was enormous. I don't know that the gap between the 13s and the Roxannes is quite as large. But subjectively, I'd say it's fairly close. In other words, it's a big enough jump that I didn't feel any buyer's remorse after hearing the Roxannes.