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
128x128mitch4t
BTW, here are the US and European equivelent #s for these tubes. 6DJ8-ECC88, 6922-E88CC,CCa, 7308-E188CC, the E288CC european tubes are physically longer in length and have a longer life span. There are also US JAN and British CV military tubes as well.
I am using Gold Aero Platinum, Gold Pin 7308 tubes in my SP9 Mk II. Sounds very much better than Valvo 6DJ8's.
Sorry for tagging on to this thread, but I also would like to know what you would suggest for tube's for my LS 25 mkl and PH 3SE. And....should we keep the pre-amp's and phono amp on (mute) if we listen 1 hour a day???

I welcome your responses!
Rick