When to replace old drivers?

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!

George:

There is a "search/notify" function on Ebay that my wife uses for odd/rare items that you may want to look into for replacement drivers.

When I posted my response there were a pair of very low mileage 166en's listed as BIN for around $140/pair, but they seem to have been sold since.

When Terry first introduced the Abby's my wife liked the look and wanted me to purchase a pair, but I balked because I had gone from 300B to 2A3 amps and wasn't comfortable that my 1-2 "usable" watts would be OK (the previous SS rectified 300B amp had plenty of driving power, but it retailed for considerably more than the Bottlehead gear).

Anyway, good luck (plus I'm envious of you recent LTA purchase;-).

DeKay

Well, I'm not familiar with your speaker cabinets, but any time yo start a DIY project like this you need to be prepared to go down unintentede rabbit holes :)