So the tube worked in both channels for one session. Next time I powered up the amp (without touching the tube at all let alone removing it) the left channel had completely cut out again... Tried the solder trick today with 30 seconds on each pin plus adding a bit of extra solder to the tip. No luck! Still only one channel. I took the tube out and gave it a dozen good whacks against my mouse pad. Tried it again and voila, both channels are working again! Very strange - I guess I'll just keep whacking it...