It's impossible to put a number on a subjective measure like that, but I can summarise the reasons I prefer the K10, keeping in mind that I am comparing a custom K10 to a universal SE846 (no SCS or similar).
The bass on the K10 is a tiny bit less in quantity, but that actually better balances the overall sound signature (compared to any of the 846's filters including modded ones). The bass quality and texture is also slightly better than the 8s. Importantly, the quantity of bass is 100% a question of personal preference and the quality of bass difference is very marginal.
In the mids I'd say the 846s are sweeter overall, but the K10s are great too - they're just not quite as forward. I honestly couldn't choose a winner here and love both for different reasons (forwardness vs overall balance)
Treble is a fairly easy win to the K10s IMO. The 8s have great treble, but there's just a tiny edge to it even with the darker filters (blue / black). The K10s have more extension and smoother delivery than the 8s.
Staging is the final area of note and while the 8s offer a good image, the stage is still quite small. The K10s have an expansive stage that is also the most coherent I have ever heard (up there with HD800s) so they are on par or better than the 8s in terms of image quality and are better in terms of stage size.
I can't stress enough that the 8s are awesome earphones and at <$1000 they are unbeatable. Comments that the K10s aren't worth the additional funds are entirely founded - the law of diminishing returns does apply here - but if you're happy spending the extra you will be rewarded with gains in almost every aspect of the sound when comparing a custom set of K10s to the universal version (non SCS) of the SE846.