Without getting into details, NO, they are not interchangeable. Not even close. You can substitute 12SL7 for 6SL7 and 12SN7 for 6SN7 (the 12 series just have a higher current capability.
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It's true that they are not interchangeable per se, but they are somewhat close (relatively).. both dual triodes, differing mu. However, 12SN7 is not a sub for 6SN7 in any way, (nor 12Sl7/6SL7) without changing the heater voltage. The 12-series tubes will work in 6-series gear. 12volt heater vs. 6 volt heater. Current does not come into play at all. (e.g.: the above info is wrong.)
6SN7 subs: CV1988, 5962 (most of the time), 6SN7 GT, -GTA, -W, -WGTA, -GTB, -WGT. Maybe some others too (depends on gear)
6SL7 subs: 5691, and all the above suffixes (-GTA, etc.)
Just to add to Ed's thread: Same in only pin configuration and heater and plate voltage - but not the same operating characteristics. 6SL7's have a higher amplification and a lower transconductance than the 6SN7. The tubes are close enough, however, to WORK in each other's sockets if the amp or preamp just happens to be designed within their respective operating characteristics (my amp manufacturer informed me that the 6SL7 can work - but not substituted -in place of the 6SN7 although I haven't tried it). IMO, if your equipment is designed around one or the other, use just that. I doubt that any amp designer would be surprised to learn that another tube than spec'd sounds better in his topology.
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.