Class A power amp will give the best BASS SLAM?

Plinius. Various models available!

The preamp plays a vital part in bass slam as well. A good preamp can give you added slam if the amplifier is up to the challenge.

A major consideration in answering your question regards the partnering speaker, as well as the rest of the components, which you didn't indicate. An amp alone doesn't slam, it's the complete system that all contributes to the end result in this area. One can have a very high quality amp that will deliver tons of current with a high slew rate and still have a compressed result if the speakers are not capable of delivering dynamically. So system context and room size all play critical roles in the final outcome. There is no simple answer based on the amp alone. What do you have in mind?