Preamp differential tube gain (12 db gain) - whats this mean?

It tells you how much gain in the pre-amp section. Too much gain mixed with very efficient loudspeakers and you might be able to hear some noise. Quality of the signal is not related to gain. It's so you can match sources with your amp/preamp combo. Too little gain and you may not have enough power to fully drive the amp. 

12 dB is what my pre has for gain....its ultra quiet.  The preamp it replaced had 25 dB of gain....noisey and the volume was too sensitive.  9 o'clock on the volume and it would blow you out of the room....  

Thx, seems this differential tube gain benefits sound in my system, both ht and stereo.  Really livens things up when passing ht sound thru this preamp.  And ht even gets better after using dirac. 
 

Which preamp are you talking about?  My experience with digital front end is that 12 dB is compatible with most amplifiers unless they have input sensitivity much lower than 1V. As per the one replaced by oddiofyl, many have too much gain which is then attenuated. With highly sensitive amplifiers there is greater chance of noise.