Sovi got the C12 and imo it sounds like an improved KZ ZS10 Pro. Personally i prefer the KZ ZSX though. The C12 is a bit more forward in the 2-5khz range, which makes vocals more forward but also more fatiguing in general. No sibiliance, just a bit edgy in the 2-5kz range.
Did you try swapping the ear tips for the star line ones in the package?
I get the opposite effect...
With the C12, and using the star line ear tips the sound is balanced with no metallic sound and truly balanced range with plenty of details and non-fatiguing sound that also shows great sonic improvement. I really due like the ZSX, but because of the bulky housing (might not be for everyone) and the way the placement of drivers are placed in their housing, I do still hear a metallic-like signature creep-up once in a while. So, it helps using the star line ear tips as well smoothing out the whole signature, but at the same time, ironically it does lose some clean details in the 2-5khz register, while the C12 presents it perfectly.
So, I have it like this...
If you purchased the C12:
The default ear tips has a bit of an upper range 2-5khz that can easily be fixed by swapping to the star line ear tips for an enriched and perfectly balanced non-metallic sound signature.
If you purchased the ZSX:
The default ear tips might be the best way to go by making sure they are pushed all the way into the nozzle itself, so when you finally wear them you get a very similar balanced sound signature, but as I said before, because of the way the drivers are placed in the housing, you might still get some metallic sound creep-up at you. So, best bet is to use the star line ear tips to smooth out the slight metallic sound for an enjoyable experience even though you'll experience a more distant midrange giving them almost a slight V-shape effect.
Lastly, on the positive side they both demonstrate incredible improvements on their drivers, sound, and appearance, while listening to a non-harsh and non-sibilant experience.
I'll express more details later in the week...
-Clear