i’d say it’s a mistake to try to generalize about tube or solid state preamps having better or worse bass. amplifier-speaker relationships would be more dominant for bass performance than particular preamps.
easier to make a case that past a certain level of gear, preamp to amplifier same brand synergy is more significant. at more modest levels of gear, for stand alone preamp; it’s case by case.
so audition other preamps in your price range, in your own system, with an open mind. or ask other Krell amp owners about it.
i do agree that preamp bass performance can be significant. for instance, at the top of the food chain, the Gryphon Apex Commander preamp does have more bass authority than my darTZeel preamp. but musically the dart preamp is more musical overall. the dart gets out of the way, the Gryphon takes control. a matter of musical priorities.
personaly i have no interest in hearing the gear signature as a non musical coloration imprinted over every recording, i prefer the music. YMMV.