the bump and rebound might not be matched, but its not vastly different to having them set differently front to rear on the same brand. Its probalby more that the spax arent that good in general, but its possible to make it do roughly the right thing even if its not perfect.
New bushes, be they poly or rubber will help.
As a short term test, take the ARB off (I know someone claims its dangerous but its not ) to see if more front roll is what you need to get the tyres to dig in.
Also make sure the camber and toe is good.