The LP stands for long-plate, the S for spiral (for AC filaments). 
There are stories about people returning this tube, because they didn't see the filaments glow and thought it was defective. People, use your ears for listening, not your eyes! But they were right, it is hard to see if the tube's functioning, I had to stick my head in my amp to see some vague glowing. And the sound? A lot like the 12AX7WA, but brighter, and that's a plus with all types of sounds. The lead still has that nasal feeling (good), the rhythm is still a little hollow (not so good) and the clean sound is very rich and clear. One time during rehearsal I noticed that this tube is more microphonic than the 12AX7WA. I bought a tube damper, but due to the LPS's slightly bigger size it wouldn't fit! The 12AX7LPS scores a better overall than the 12AX7WA, but they both have problems with rhythm.
There are stories about people returning this tube, because they didn't see the filaments glow and thought it was defective. People, use your ears for listening, not your eyes! But they were right, it is hard to see if the tube's functioning, I had to stick my head in my amp to see some vague glowing. And the sound? A lot like the 12AX7WA, but brighter, and that's a plus with all types of sounds. The lead still has that nasal feeling (good), the rhythm is still a little hollow (not so good) and the clean sound is very rich and clear. One time during rehearsal I noticed that this tube is more microphonic than the 12AX7WA. I bought a tube damper, but due to the LPS's slightly bigger size it wouldn't fit! The 12AX7LPS scores a better overall than the 12AX7WA, but they both have problems with rhythm. Information from http://www.fbk.eur.nl/BIT/MHM/gear/preamptubes.html
