When to replace old drivers?

If they sound fine leave them alone! Buzzing is caused be a rubbing voice coil - not a speaker cone! 

Yes, don't let audio paranoia get to you. Leave them alone and enjoy.

I doubt whether you could ever hear it's effect but if it makes you feel better you could remove the driver and VERY carefully iron out the crease as best as you can.

If you haven't already done so.

I'm sure there must be plenty of advice on how best to do this with Fostex drivers.

Those whizzer cones are just irresistible magnets for inquisitive fingers. 

For peace of mind you could play a mono track and switch from left to right checking for any significant difference. 

I still remember being almost a nervous wreck when I used to take our old Datsun on long trips. I soon learned not to be overly concerned with the million and one things that could possibly go wrong with that car. 

As for flying...

They are probably fine (my wide rangers are 50+ years old and still sound good).

However, you might pick up a backup pair as the 166E and the Sigma are now rare and pricy (if you can find them @ all).

Check Ebay (now) for the EN model if you think it will work.

Did you ever try the other drivers (165 something)?

DeKay

roberjerman & cd318, thanks for the encouragement to leave well enough alone. I wouldn't trust myself to iron the whizzer, but getting a backup pair of Fostex is an appealing idea, DeKay... looking around, the 166e and more recent 166en are indeed hard to find (the 165k is for nearfield I believe). Thanks for the help!