The only way to "thump" the tank is with the jacket off. Field service technicians who do commercial softening repair usually do about 6-8 repairs a day. The first thing we do when we walk up to a system is kick the brine tank and hit the tank. Commercial systems never have brine tanks, when they do, the health inspectors go nuts because of the potential for little critters to hide. Residentially the tank jackets are there for mostly aesthetic reasons. Regionally jackets can be used for more than just decoration, but here in California, they look real neat. We even manufacture an aluminim diamond plate jacket.
Cold water is denser than warm water and lifts the bed much more than warm water. 2-3 sizes down is typical for Clack up-flow systems, but... if your water temperature varies significantly seasonally, or you pressure varies a lot, upflow can be a little more difficult.
Adjusting the brine draw is done with the injector only. You could easily replace yours for less than $15, and it is very easy to do. The Clack valve has a threaded cap on top of the valve, the injector is underneath. . The injector can easily be removed with a pair of needle nose pliers.
Honestly, even if the resin bed moves during regeneration, the system will likely be fine, it just lessens the potential for a perfect regeneration.
The difference between a perfectly regenerated resin bed and a "oh so close" regeneration will only be a part per million or two for quality, and a couple percent for capacity. The reserve capacities, variances in incoming hardness, etc will all outweigh the difference.
Hope that made sense. I have been installing my new system all day today. I will post pictures of it in my Prototype thread.