Searching for this question I have ran into this. I was going to change my new tube set for my Cary mono block 2A3 SE. I found out that the preamp tube 6SN7 was replace by 6SL7 by a previous owner. Both blocks was working fine since I have them. I am not sure if this should be ok but I rather change them back to normal 6SN7.
Please advice.