I use heavy Pyramids on my SX P bass with a Klein Epic '58 pickup.
I order custom sets direct from Pyramid. .043 .065. .085 .110. They definitely feel more substantial than the lighter sets. I used to use a custom .055 .075 .095 .115 set but they were slightly to taut for me.
They are round core per my request (only their electric guitar flats are normally round core).
The E string will take some time to settle in. At first, mine sounded too muted. I took it off a couple times to experiment with other strings. When I came back to it, about the third time, it seemed it was ready to relax (it's also possible that it had a slight twist that needed to work itself out).
These now feel and sound like the Goldilocks set for this particular bass, and my playing style.
I am heavy handed and like to spend most of my time plucking over the end of the fingerboard. I can get some throaty mids, and a really pronounced, percussive thump. Playing over the pickup, or between the pickup and the fingerboard, gives me that big, round, chocolaty, 1973 John Paul Jones at Madison Square Gardens sound.