Re: Major differences between 59s and Seths?
I've played both in my Les Paul.
The '59s have a bright top end and solid bass, whereas the Seths have a smoother more mid-focused tone that in turn gives the impression of a softer bottom end.
That's not to say that the Seths aren't bright, but they're less bright than the '59s.
They're both really nice pickups if you're looking for vintage humbucker tone.
This is a huge generalization that totally ignores other gear and technique factors, but think of Jimmy Page and Paul Kossoff. I'd say Page's tone is like a '59, Kossoff's is like a Seth.