So, I've found in one of the local shops in my town an old Fender flat E string! I blocked at the right length past the nut the outer coil wrapping with some red sewing cotton and Attack superglue; after that, I've cut the string excess and removed the outer metallic flat wrap up to the red cotton. The end result is satisfactory. Even output level from all 4 strings and finally good sustain on the E string. Yes, I know: I've cut the string core too short... but it works anyway... 