Would you buy a 12-14 year old Integrated?

Problem with used Gryphons is that you can almost always find them only in Europe. I saw a few previous generation Diablo for around $5k with phono stage - incredible bargain. I guess, you can run them with high quality step up transformer, or have Gryphon change the voltage. I would first try the transformer. Gryphon uses military grade parts, I wouldn't worry about the age of Gryphon if it is, say, under 25 years, and might take a risk even if over it. With some service, once or twice, Gryphons should last a lifetime.

I have a 16 year old Redgum integrated and for the last five years that I've had it the sound is exactly the same. Well, that was wrong, the sound is better because of upgrades in other parts of the system. I am going to send it to Australia for a check-up and to replace the input selector switch that the previous owner played too much with. I expect it to last trouble free for another 7-10 years.
I don't know McIntosh, but if 7900 sounds better and you can manage extra $800, that would probably be the preferrable choice. Unless for whatever reasons you want the 6900, of course.
I also have 18 year old Audiolab integrated that I no longer use, just play it for a few hours every year, the rest of the time it spends in the closet.