A good sonic upgrade would be Amperex White Lable USA 6922 or 7308 PQ variety
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I used many a tube variety in my previous LS15 and LS25. The ARC's sounded best with Siemens 7308 circa pre 1970. Then I tried some of the Cryo'd SLNM 6H23n-eb from ATSI. Ken Chait cryos these tubes and the results are nothing short of amazing. They have a sonic signature that resembles the Ediswan 6922 I tried and liked..but the Cryo'd 6H23n's are more transparent and more dynamic. Soundstage goes very deep which is something that none of my NOS tubes, Ediswans, Amperex PQ's, Siemens CCa's or Mullards could do. At $74.99 a pair..you should check them out. www.tubeman.com/item224.htm
I have an LS7. Have tried NOS Amperex (both Bugle Boy and orange-globe-logo). Nice -- but, though this may sound heretical, I still prefer the stock tubes, which are Electro-Harmonix 6922, available from just about any tube dealer. (I get good service from http://www.tubedepot.com/) Maybe it's all just a matter of the particular combination of power amp and speakers with this preamp, but in the end, I prefer the greater transparency and detail of the EH to the sound of the Amperex. I think that puts me in a minority, but that's my experience.
For more recommendations on 6922 / 6DJ8 tubes, check out Joe's Tube Lore at www.audioasylum.com/audio/faq/joes-tubes.html