Is there an average life span for a preamp?

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!

Yes, but it is unknowable for each particular case.

I've had my Luxman preamp for over 35 years without any problems whatsoever.

The main limitation is the life of the filter capacitors in the power supply. These days the half life of them is about 20 years (IOW- in 20 years half of them will have failed).

Of course many units will run longer than that but its a safe bet that if the filter caps are changed out they will sound instantly better!