One of the quieting/stealth features on the future Columbia class SSBN(X) is a possible heat exchanger quiet mode in its proposed S1B reactor. This reactor is also called Columbia Program Element PE0603570N/Project 3219 (see page 32).
Apparently the S8G reactor on the current Ohio SSBN already uses a heat exchanger. I think it logical that the heat exchanger aspect will be carried over to the future Columbia.
Minimising moving parts contributes to quiet mode. Such a quiet mode may ideally be available when future Columbia's are operating at their (about 5 knots?) on station cruising speed.
If all of these features are in the Columbia's and perhap future US SSN(X)s they may make them quieter than SSKs on battery mode.