Ride height on the Hoyle suspension is easily adjusted by winding up and down the lower spring seat on the damper with a C-spanner. Be aware that there is a locking grub screw in the threaded ring that must be slackened before attempting to wind the ring up or down. I have recently been adjusting mine to get the steering tie rods parallel to the ground.
Slackening off the dampers might help a hard ride, but altering the spring rates is the best solution. I wonder what springs you have? I have tried several different ones. For the rear on my GTV8 it seems that the standard 10" long 300 lb/in spring is the right one for me. On the front I started with 9" long 300 lb/in springs which were too hard in my opinion. I felt that the front of the car kept bobbing up and down, as if the springs couldn't compress at all when a bump was encountered. So John Hoyle sent me a set of 9" x 200 lb/in springs. These were better but probably a bit soft, so I now have 9" x 250 lb/in springs at the front, and the ride is good. An MGB is never going to ride like a heavy modern car. It will always react to bumps, but mine is now quite comfortable and has stopped the oscillating movement.
I see from the latest Hoyle web site that the standard springs now supplied are 10" x 300 at the rear and 9" x 250 at the front, so by chance I have settled on those recommended items. I suspect that a 225 lb/in spring might be a slight improvement for the front, but I don't have a set of those. You will need a spring compressor to remove and replace the springs from the dampers.
The springs can be bought quite cheaply from suppliers like Merlin Motorsports (https://www.merlinmotorsport.co.uk/s/su ... er-springs
). They are standard 2.25" ID springs available in a wide range of ratings and lengths. Or you can buy them from Mark at Hoyle. Back in the days when I set mine up, John Hoyle used to send me springs to try free of charge, which is why I have three different ratings on the shelf.