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