A quick extract from my German review I wrote many months back and translated it to English for Head-Fi, with some additions:
Resolution, extension, naturalness and soundstage are a distinct upgrade with the 846. Though, sound signature is different and the 846 focuses more on the sub-bass and sounds a bit thicker in the root and mids.
This plot should give you a rough idea on what the differences between both IEMs are.
Please note that it wasn’t recorded with professional equipment but with my Vibro Veritas coupler that was pseudo-calibrated to more or less match a real IEC 711 coupler’s response with applied diffuse-field target, hence the results shouldn’t be regarded as absolute values but rather as a rough visualisation. Especially at 3, 6 and 9 kHz, there are sometimes greater deviations from professional plots – but for a general, rough comparison between various in-ears and a rough idea of how they sound, the results are sufficient.
green is SE846 with treble filter, yellow is SE425 (both IEMs were measured with the silicone tips)