I finally changed the tubes this weekend -- my first experience with this.
After posting this thread and reading your responses, I think I was determined to burn out my old set to better understand how tube degradation sounds.
Having purchased the SP11 in 02 (?) I was thinking that the tubes were at least 3 years old.
And although I often left the system off for days at a time, there were other times when I left it on 24/7.
For the last 6 weeks or so, the tubes have also been left on 24/7.
I am still curious to think that they didnt sound that bad, other than some occasional squeaky sputtering and some bloat in the bass.
My main curiosity pertained to the fairly flat image that I have and I wondered if new tubes might increase the illusion of depth in the soundstaging.
The bad news is that the image is still pretty flat, something I hope might be changed with new cables.
The good news is WOW -- subtle in a way, but ultimately a huge improvement.
Everything seems much more nuanced, more air and separation between instruments, breathier detail with vocalists, no more pops or squeaks and bass seems tighter and more defined with less boomy bloat.
I guess I needed new tubes?!