six year old 6sn7's, pushing my luck?

You should buy yourself a backup set of new tubes. The inexpensive EH brand would do just for situations like this one. Rotate the new EH tubes into the preamp and listen. If these generic/boring sounding tubes sound better than your current tubes, then you will know it's time to replace. Tubes as you have stated, do gradually wear out at such a slow rate that it is quite possible for it to happen without you noticing it.

My first thought wsa why not shut it off except when listening, but then I thought maybe leaving them on is helping them survive. Everything I cna remember reading says to shut tube gear down when not using, but leave it on if it will be unused for only a short time. I remember - vaguely - something about the heating and cooling cycles not being good for tubes, maybe the expansions and contractions of the parts damages the coatings).

The sophia tubes run $100 each and are pretty good. You might want to think about them. I also agree it makes sense to keep a cheap pair of current production tubes around.

If you are using single ended inputs, you only need to change the 4 tubes on the left. The 2 on the right of the main group (not the headphone ones) are used as buffers for the balanced inputs. In a recent newsletter, Cary had a picture of a modified SLP-05 they had at RMAF with 4 Black Treasures and 2 more normal tubes. People like the Black Treasures but they do not fit the holes in the top plate - you need to enlarge the holes. New Psvane tubes (CV181-T) are based on the old Treasures and are starting to become available from Grant Fidelity, at half the price of the Pope's. I thought about the Pope's but Upscale says they only sell them to people you got their pre from them. But, it sounds like your Pope's may last forever. Good luck!

6SN7's are extremely resilient tubes that were built for applications like televisions - it is very common to get 100,000 hours or more out of them - I wouldn't worry about them if I were you.

The nearly universal belief nowadays is that tubes must be turned off when not in use to preserve tube life. It's in fact more complicated than that. Output tubes used in power amps have to be turned off when not in use because they pass a large amount of current. Small-signal tubes like those used in tubed preamps and DAC's, however, actually last longer in most circuits if they are left on 24/7, as the voltage rush that occurs when tubes are powered up stresses them, and the thermal cycles they experience, whereby they got hot when powered up, and then cool down when powered down, likewise stresses and weakens tubes after numerous cycles. The key question is how close a tube is being run relative to its maximum voltage - most preamp circuits run small-signal tubes at relatively low voltages, meaning relatively little heat - in such cases, it's best to keep them on.

If the preamp uses a tube or tubes in the power supply, however, it's more complicated, as such tubes can be stressed like output tubes - it depends on the circuit.

The Colossus computers used in World War II to decipher enemy radio transmissions used thousands of small-signal tubes. The Wikipedia entry for "vacuum tube" has this to say about operation of the computer:

"The Colossus computer's designer, Dr Tommy Flowers, had a theory that most of the unreliability was caused during power down and (mainly) power up. Once Colossus was built and installed, it was switched on and left switched on running from dual redundant diesel generators (the wartime mains supply being considered too unreliable). The only time it was switched off was for conversion to the Colossus Mk2 and the addition of another 500 or so tubes. Another 9 Colossus Mk2s were built, and all 10 machines ran with a surprising degree of reliability. The 10 Colossi consumed 15 kilowatts of power each, 24 hours a day, 365 days a year—nearly all of it for the tube heaters."

The Wikipedia entry for the Colossus emphasizes this point:

"Colossus used state-of-the-art vacuum tubes (thermionic valves), thyratrons and photomultipliers to optically read a paper tape and then applied a programmable logical function to every character, counting how often this function returned "true". Although machines with many valves were known to have high failure rates, it was recognised that valve failures occurred most frequently with the current surge at power on, so the Colossus machines, once turned on, were never powered down unless they malfunctioned."