Jay:
You've checked other parametres I assume (wires, source, etc)?
I m no expert but in general tube amps compared to a SS *lack* bassActually it's the interface with the wrong spkrs that does that. BUT, anyway, you're speaking of the pre. Normally, there shouldn't be problems at one end of the spectrum (i.e bass) just because yr pre uses tubes. In fact tubes are quite good at amplifying small signals. It may be a characteristic of that particular pre -- but this is unlikely.
You've checked other parametres I assume (wires, source, etc)?