Kit,
If it still sounds good to you other than the hiss in teh one channel, try rearranging the tubes in the sp16 in different orders first before replacing.
SP 16 uses 6 tubes. Tubes age at different rates. Location of the tubes in the sp16 slots help determine resulting background noise levels I found. As tubes age differently, moving a "better" tube into a different location can make a difference and buy you time before you are inclined to replace.
If it still sounds good to you other than the hiss in teh one channel, try rearranging the tubes in the sp16 in different orders first before replacing.
SP 16 uses 6 tubes. Tubes age at different rates. Location of the tubes in the sp16 slots help determine resulting background noise levels I found. As tubes age differently, moving a "better" tube into a different location can make a difference and buy you time before you are inclined to replace.