What tubes to use for Audio Research SP-9 MkIII?

I have never opened the case on my AR SP-9 MkIII preamp. I called the guy who sold it to me and he says that it has Sovet6922 or called 6DJ8. A pair of Simens old new stock tubes 6922 made in Germany are for sale for $130 bucks...is the price of the Siemens tubes a good deal? Will the change in tubes make a significant improvement? I've never opened the case...how many tubes will I need? The AR users manual suggests leaving the preamp on all the time....how long will tubes last if the unit is always left on? Thanks,...Mitch