The traditional Morgan front suspension is indeed a strangely complex beast. Because the springs are contained top and bottom with the stub axle in the middle, the main and rebound springs act together in parallel, so the spring rates are added together.
At first glance this is counter-intuitive, some people believe that because the rebound spring is pushing upwards on the stub axle, it is acting to reduce the spring rate on the main, whereas of course it's actually doing precisely the opposite.
MMC have done a lot of work recently with their suspension supplier. Watch out for rising-rate springs coming soon to give the best combination of supple control over small bumps, firming up quickly to stop bottoming out over big lumps
