As an ADS vintage fan with L910's and L710 currently, both have had their crossover redone and both have new tweeters and mid drivers from Richard So, there are parts still available.
These were designed by the the same person who designs Arial Speakers T10 etc.
ADS are wonderful sounding designs with many innovations for the time, obviously those designs are older now though.
They are also very good at imaging and don't have an issue with brightness as a whole they are fairly neutral for the era they were designed. i love the dome midrange and tweeter in them. I'm not sure what the efficiency of the 1290's are i know my L910's are 93db 4ohm. not hi enough for 300b SET that's for sure maybe a parallel one at lower listening levels.
that said the Vandersteen 5 is a more modern design and will do somethings that the ADS wont be able to do as well. from what i remember its been a long time since i heard any Vandersteens the v5's will be a better overall sounding speaker but not drastically so. and you may prefer the dome mid of the 1290's better. I'd say the ADS will be smoother sounding but may not have the detail level as the V5's.
neither will be a good match with a 300b based amp IMO. unless your only driving the top end with the 300b amp.