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

I would NOT leave the linestage on all of the time. That will shorten the life of the tubes as well as other components (heat shortens the life of capacitors and resistors).

Your Cary has tube rectification which provides a soft start turn on of the tubes. There is no benefit, in terms of preserving tube life, to leaving the gear on all of the time. There is also a small risk of fire from leaving gear on unattended.

Small signal tubes, like the 6sn7 can last quite a long time, provided that they are run well below specified maximum parameters. It appears, from your experience, that that is the case with your Cary linestage. Still, with valuable tubes, it makes little sense to use them up heating your room--they are NOT making new tubes that are comparable to some of the better 6sn7s of yesterday.

Mr T- My Cary monoblocks use 6SN7s as phase splitters & drivers. I tried the top ten 6SN7GT/VT231's(all bottom gettered, but for the Sylvania 'W' tall bottles, from the 40's), mentioned on the Chimera list(http://www.dehavillandhifi.com/6sn7_vt.htm) in them, the first year of ownership(2002). I found the cleanest, most liquid, least tubey(to me=unnaturally warm) and most extended(both directions), to be the four that I mentioned as having in reserve. The Sylvania and Ken-Rad VT231/6SN7GTs only seem to give up a bit of sound stage width & depth, to the TS's and 'W's. That's using mixed pairs, in the mentioned positions. I tried every possible combination, in that year.

Dearing is correct about the care of small-signal tubes - provided they are not run in a circuit at near maximum voltage, which exposes them to a lot of heat, they will generally last longer than they would if you turn the unit on and off.

That said, Larryi mentions that the Cary has tube rectification. As Dearing notes, tubes in a power supply can be run hard and this complicates the question of whether to run the Cary 24/7. In addition, Larryi says that the Cary features soft-start, which, assuming proper functioning, eliminates the damaging voltage rush that occurs at power up.

The best way to determine whether 24/7 operation is best in this unit is to begin by retubing it - that way, you are starting with tubes that have not been damaged by previous on/off operation. Then just try it. Kevin Hayes of VAC, who is generally an advocate of 24/7 operation of small-signal tubes, explains that when run 24/7, small-signal tubes either die within the first 250 hours or so from "infant mortality" or they last basically forever. Consider the following from the "TIPS & ADVICE" section of the owner's manual to my VAC Rennaisance 140/140 Mk. III tube amps:

'How long should tubes last? It has long been known in professional circles (and probably now forgotten) that a tube such as the 12AX7 will display BETTER performance characteristics after TWO YEARS of CONTINUAL operation than when it was new. In normal use it is not unusual for a low level [small-signal] tube to last 5 years or longer. Output tubes [i.e., power tubes used in tube power amps] are another story, as they are continuously providing significant amounts of current.' (Emphasis original).