I second the questions about what source(s) you are using. In particular because the rather low 10 Kohm input impedance that is specified for your preamp will definitely roll off the deep bass if driven by the many tube-based source components (and perhaps also some solid state components) which have output coupling capacitors that have been chosen with the expectation that preamps having higher input impedances will be used.
Regards,
-- Al
Regards,
-- Al