Tubes for Audio Research LS-15

I recently bought a used Audio Research LS-15. I love the unit, but I am interested in upgrading the tubes. Any suggestions?
I have the LS-15 and recently got some Siemens 6922s and some Sovtek 6922 old stock tubes as, I hope, an upgrade. You can contact Upscale Audio (website is or for recommendations and purchases. Also, there is currently an auction of some good 6922s on Audiogon, I think they are Siemens too. How do you like your preamp?
Vintage Amprex Bugle Boy 6922, can be bought on Ebay, there will be used but do sound great......
Thanks for posting, I have a LS-15, it is a great preamp. I will be re-tubing soon. Glass audio can lead you to a lot of tube info.