I had the same pre for about 10 years. Initially I left it on 24/7 and changed out the tubes very 9 months or so. Later I did like Thorman, turned it on when I got up and off when I went to bed. I also usually got different tubes for the one's associated with the phono circuit but you don't currently have that issue.
The flat imaging is probably a function of your cables and your amp, assuming you still have the 23.5 which while extremely quiet and powerful, never struck me as an imaging champ. Good for its day and lots of slam for those Maggies but maybe time to look for something different. FWIW, I replaced my SP11 with an ARC LS-15. The LS-15 was much better at the frequency extremes, imaged better and had greater dynamics, ie no contest.
The flat imaging is probably a function of your cables and your amp, assuming you still have the 23.5 which while extremely quiet and powerful, never struck me as an imaging champ. Good for its day and lots of slam for those Maggies but maybe time to look for something different. FWIW, I replaced my SP11 with an ARC LS-15. The LS-15 was much better at the frequency extremes, imaged better and had greater dynamics, ie no contest.