I thought I'd re-visit this thread after three and a half years. There's some good, relevant information in the OP, but I've also had a chance to listen to a lot of speakers in the meantime.
I worked out on a of AR-3a in an "ultimate restoration". The refoamed AR-90 were clearly superior, and soon led to me buying a pair of AR-9. A year later, in a tight spot, I sold them.
While I was never 100% satisfied with the treble from the 90 or 9, they trounced the early AR speakers I've owned and heard in every way. Combine that with my belief that the early AR paper dome tweeters are all suffering from materials failure, and the writing is on the wall for me.
While I admire the history of AR and think the original line are handsome mid-century artifacts, I couldn't see myself using anything earlier than Teledyne ARs as my mains.
That said, I'd gladly give another pair a try. I honestly wouldn't mind fining another pair of 90 or 91, and might commit the blasphemy of trying to re-engineer a different tweeter.