Is there an average life span for a preamp?

Joe - 12000hrs is nothing. The only perishable things inside (other than mechanical switches) are electrolytic caps. Their life is in order of 50years in room temperature. Every 10deg C temp. increase cuts their life by 2 (they dry-out). Since preamps tend to run cold I wouldn't worry for at least next 20 years.

Pre-amps are one of the most trouble free or reliable components of a system.
I've have some friends who has 20 year old pre-amps; while some needed re-capping, others works just fine with just regular tube changes.
Amps on the other hand is another matter due to heat & other variables.

My impression is that it should last a long time. It seems that you all feel the same way and that is good to hear. This particular component is class A and it does run warm when it is on. Does anyone feel that this makes any difference?

AFAIK every preamp is class A. Saving power (class AB) is unimportant in preamps.

Joe,
I've had the same pre-amp for going on 13 years. I have not had one bit of a problem. I can't believe how good that little thing sounds. Unlike you, however, I'm not really fond of the "passive" mode. But I'm glad that you like it. I can't believe Adcom stopped making the unit. Find yourself a reputable tech and put him/her on standby. If it ever needs servicing, it should not be a major deal. If you live in the Hudson Valley area, let me know. I can recommend a good technician.

In the meantime, keep enjoying that little giant killer!