I know the sound your looking for exactly!
String that baby up with GHS Precision Flats!
They make your P-Bass sound the way they sounded in the
50's and 60's. Use the 45-65-85-105 set!
If you string through the body you'll need extra longs!
Well the Roto's were okay, liked that waxy feel, but they had a certain "tweak" in the sound that is not well complimented by the single coil pickup.
I put on some lighter gauge GHS Precisions and they're a great improvement for me. Right out of the box, there's a lot less harshness. Even with a pick! The 45-95 set seems pretty light in tension - even had to loosen the truss 1/4 turn. I will definitely try something heavier next, but I can "deal" for a few months.
BTW for the GHS Precisions, the "long scale plus" works just about right. Thru body seems to add about 1/2" over the standard top-load, so any strings rated for 35" scale should fit.