You have two elements that "take no prisoners" - the Decca and the Yamahas.
I know as I have both!
However the Rock should be a bonus. Have you tried adding mass to the the Alphason, or adjusting the amount of damping?
Admittedly I use an OL modded RB300 and a retipped Decca Blue but records that sound poor with the Decca also sound poor with my other cartridges.
Experimenting could be worth trying.