Time for new tubes.. which ones?


My TAD-150 preamp which has only 2 tubes stays on all the time, so every year I have to replace the tubes. This is my first time replacing them. Does anyone have an opinion as to what tube to use, or should I just use the same replacement ones from TAD? I would like to improve the sound if possible with new tubes. Thanks
koestner
If you tell us what tube types they are (not brands, but tube #s, i.e., 12AT7, 6922, etc) and what brand is in there now, we can probably help. Otherwise, no chance.
So Sorry, they are 12AT7 the preamp manual says I can also use 12AX7 or 12AU7 as well. All I know is that the original tubes came from China. Thanks for any help.