When to replace old drivers?

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 :)

DeKay, I think I was the one who got that pair--great price, and when I saw how few of them are left out there... Good insurance. Yes, the LTA is a fantastic amp; although the ZOTL10 is listed as "power amp" it has a line stage an works just fine without a pre: more than enough for the Abbys. Thanks, everyone, for helping!