I don't plan on playing them so loud, so I think the tweeters would last forever on my set if I get a pair. I will certainly not buy them used, unless they have a warranty, based on all this information you provided. It was really helpful and probably solved this "harsh treble" mystery. It was very odd how some said the treble sound smooth and soft, then a few out of the blue "its so harsh and distorted" comments. That's why I asked around here.
Yeah, based on measurements, I eliminated hype and narrowed down my choices to the Neumann KH310 and the Genelec 8341A/8351B's. The ATC's are really something special in tonality, but I feel they are overpriced. The Neumann's are half the price and there's a point of diminishing returns here. If I'm happy with the Sennheisher HD650, I will be happy with the KH 310, right? Excuse me if I'm a bit redundant here. Thank you for your time on the measurements and insight.
In my mind all comments that the treble is harsh is either that the tweeters are damaged or that they're playing them too loud and should have bought something more powerful...they sound best when not driven too hard.
Follow my advice and start with the switches in the 94dB position and maybe dial the trims back a bit and you'll be fine re tweeters. If you're using an interface for example calibrate it so 23bit (-6dBFS) is about as loud as you'll ever need them...maybe -12dBFS...but no more. If these were stolen, I would probably buy them again...
I don't think ATCs are worth it...if you're thinking of spending that kind of money go for the 8351s. "If I'm happy with the Sennheisher HD650, I will be happy with the KH 310, right?" I think so, I don't have HD650, so I can't answer that honestly.
"Since you have a lot of experience with both the KH 310 and KH 120's, how similar is the voicing/tonality between the two? I'm not concerned about bass, I can always add a sub. I'm under the impression that in order to get the stellar mids of the KH310... that I need to actually get the KH310."
I've had the KH80s too! If on a budget I would go for the KH80 and the 750DSP sub...the KH80s are great value for money...but seems you are considering much more expensive stuff I would say go for 310s and the 750DSP sub...my opinion is that the KH120s aren't as good as the KH310s or the KH80s...
If I had the money in my bank account and was buying new...I'd probably buy both the KH310s and the 8341s from Thomann at the same time, and then send one set back before the 30 day return! (I think you might actually have to do it in 14 days, there's some finer print, but that's enough TBH).