Tube rolling....

Greetings all - I just purchased my first piece of tube equipment after 20+ years of solid state. I purchased an Atma-Sphere MP-3 preamp around 4-5 years old. Tubes have never been changed. Any suggestions on tube source and type for this unit? Regards. Joe
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Hopefully Ralph Karsten of Atma-Sphere (moniker:  atmasphere), will chime in with a good recommendation. He is a regular on these forums.
Yes being a tube-newbie I need to obviously make the correct decisions.
Here's a link to an older Audiogon Forum thread asking same question (there are other threads on here regarding your preamp as well just need to type in the amp name in the search discussions box up top