I made a long and detailed post elsewhere. The high points are:

The new model is moar better. More extended on top, better detail throughout the band, better background (or microdynamics), removable cable and nicer physical design.

They need a crapload of break in - a couple hundred hours at least - and as they break in, they may actually sound worse than straight out of the box.

I used a Richard Cheese album as an audio reference.

I was a hopeless fanboy for the 420, so if you don't like the 420, don't listen to me.

Shure SE-420 vs SE-425 (long) - Head-Fi.org Community

Thanks for the review. One of the IEMs that I was always curious about but never heard was the 420s. Great that they improved upon it

i personally find the idea of little balanced armatures needing hundreds of hours of roller coaster-esque mechanical break in to be highly dubious.

Some stuff doesn't seem to change much at all and some stuff does. Beats me why. In this case, I had the broadly similar 420s at hand, so the relative change over time, one against the other, was obvious. I never have bought the 'getting used to it' theory. That was controled out here in any case.

What I find more dubious is that you used Richard Cheese for breakin ... is it really that full range?

Sent from mah Droid

Naw, for break in it's gotta be latin disco. That's a must.

Richard Cheese was a reference. It's all about the intellectual and emotional connection with the music, you see.