Riis is right, the multiple bands of the graphic EQ don't refer to the different strings. Each string produces multiple frequencies at the same time, plus each string has a whole range of notes, so (broadly speaking) each band of the EQ affects all strings.
There's not a whole lot of difference between a preamp pedal and an EQ pedal, just in general/generic terms. Both have multiple gain stages, both have various filters. The differences are only in the SPECIFIC pedals. So you can say "I like X specific pedal for Y specific purpose", but X could be a preamp or an EQ.
Most preamp pedals have only one to three bands of EQ (again, affecting all strings) so some people like a little more targeted control over the EQ. But then maybe your specific preamp pedal has a DI output, or an amp simulation circuit, or some other feature that the EQ doesn't have.