Can we first admit between us that a "former manager of an audio store" i.e. a frigging salesman is NOT the ultimate authority on speaker performance? Anyone can post a review on that site, doesn't make them a legit source in my book. I would not be surprised at all to find that they had the tweeter levels cranked to the max (louder in stores usually sells faster, esp. back then). He goes on to say in the same review that the L1590 "really needs a sub" which is fairly idiotic, in fact.
As to the rest, that would be a great argument, BUT nobody used L166 to mix records professionally, they mainly used the pro monitor series stuff which at the time had a cone tweeter of limited HF extension, i.e. the 4311 (L100 was the consumer version). I say that as a guy that was working at a store (and not in sales, either) that sold JBL consumer and pro lines in 1977, when a lot of this stuff was current.
The L166 is a fine speaker, for sure. It has - measurably - a better tweeter and better woofer than the 4311 you are using as a reference point for your argument. It's generally regarded a better speaker for home audio reproduction than the L100/4311, but those advantages also mean that you are NOT listening to a record using the same speakers it was mixed on.
The L810 are not as close to the L1290 as they are the L1230 (which I also own) but it'll still make an interesting A/B assuming both sets are in good working order which isn't a given at this age, and that the set up in the room is correct for both. I found the L810 to be sensitive to placement and didn't care overly for the bass response - a little "boxy" compared to my L710, L96, L112 etc. but others here have suggested they do better when placed optimally.
John
