Keats...by definition, you'll never find a shock that has a fixed preload for all loads. On a standard, exposed spring type shock, the preload, or sag, is set via tension on the spring. With air shocks, it's just done with air, that's all. Either way, the preload must be adjusted for different loads...well, it doesn't HAVE to be, but if you don't want to bottom out the shock you do.
A shock like a Bitubo has three adjustments: Sag (preload) which is adjusted by measuring how far the bike "settles" with any given load on the shock and is adjusted by turning a collar, increasing or decreasing spring tension. Rebound damping, which has an adjustable range of 1-5, and effects the rate at which the shock piston moves downward after a compression stroke...on a Bitubo, you can usually set this on 2 or 3 and leave it. Compression damping, which, on the Bitubo shock, is adjustable via a knob on the gas canister that controls a needle valve, effecting in a non-linear fashion the response of the shock piston to an upward stroke of the swingarm. That part is easy to adjust. The difficult adjustment is the spring preload, but once you figure it out, you'll typically have three or four settings: solo; solo with a load; two up, which may be the same setting as solo with a load; and two up loaded.
On a bike that is not a bagger, adjustment of a spring type shock is not a huge deal. But on a bagger, you pretty much have to remove the bags to adjust the preload, which is a bit of a PITA. That's what is so nice with a system like the Arnott or Legend...you push a button to adjust the sag (preload).