I'm no expert but will nevertheless add my consumers experiences. First of all, better resp. adjustable dampers could be the way to start, because better dampers can effectivly "help" the leaf springs not to bottom out, and it is a good solutions for two reasons. 1. it is a comparable cheap change and 2. you benefit of better dampers any way significantly in terms of ride quality even if you change the springs afterwards as well..
Now i come to the point why I say, good dampers "better" the behave. It's the spring itself which is the major cause.
On my 2014 4/4 it is a threshold situation, the springs are bottoming out on the straight, but only with two passaengers and a full tank So I kept them and the AVOs really help.
(I will go for asymmetrical leaf springs in the future for another reason: anti torque behave, and hopefully the new springs are at least as stiff as my current ones).
A word re Panhard rods. As a layman I can only say the Panhard rod which was installed into the P4SS by the factory worked excellent. Why excellent? It did what it should do, avoiding lateral movement, but it did not add this certain hardness and delivered a quit smooth ride without those nasty bumps linked to your back. I had added Panhard rods on two roadsters but the drive was very unpleasent even if the rod did his main work propperly. I removed it very soon again and added a second anti tramp bar instead which "helped" against lateral movement.
As has been said a Panhard rod generally does not helpt directly against bottoming out. But it could "help" to make the ride less comfortable.
My 4/4 should keep simple and lightweight. I don't see the the need of a Panhard re the way I drive her but the AVO shocks help a lot.
If your springs are way to soft I also would start changing them but as I said the AVOs are an additional improvement anyway.
