I have owned (still do) the U10, U12, and MX-U8. They all have good VFM, but sonically I still prefer the older Stello U3.
For whatever reason the U3 just sounds more melodic, with a noticeably bigger (read: deeper & wider) sound stage presentation.
I have added internal EMI damping and upgraded fuses to each of the U10,12, & MX-U8, - which has helped somewhat... But they all still fall a little flat sounding compared to the U3.