What improvements would I get with a "better" preamp?

@rockadanny What tubes are you using? Have you done any tube rolling? Different tubes can make quite a difference in what you hear. Bad tubes can make a really nice piece sound pretty bad. Just a thought. 

billpete, “blind” tube rolling could be really confusing, if no specific design fundamentals’ checked (manufacturer’s voltage map). even excellent tubes can sound bad in design, which was optimized for specific tubes (load line centering, loading, biases). 

@westcoastaudiophile Not sure what you mean by "blind". If the tubes are the same nomenclature as what your preamp was designed for, what is "blind" about making some of your own comparisons?

@billpete “blind” means rolling tubes w/o understanding tube amp design fundamentals, "same nomenclature” tubes over decades have large distribution of key tube performance metrics, that’s why! designer uses tubes available in volumes, at the time of design, to guarantee performance, thus extra step such as verifying voltage map is necessarily. 

@westcoastaudiophile So............I can't just listen to see how they sound to me?