I certainly hear no harshness or crossover anomalies with my Falcon LS3/5As, they are absolutely seamless across the mid-band to my ears. I just can’t hear where the bass-mid stops and the tweeter comes in.
There is absolutely nothing in common between the Harbeth and an LS3/5A. Literally nothing aside from basic cabinet dimensions. Absolutely everything else is different (drivers, driver materials, crossover design, impedance, cabinet materials etc etc). No commonality here. They are as different to an LS3/5A as they are to the ATC.
PS How loud do you listen and with what amp do you drive them? Both the Harbeth and the LS3/5A have a limited volume envelope. The ATC will comfortably go a lot louder assuming a good clean amp behind it.