I'd think removing the acoustic lens from the tweeters has removed some comb-filtering effects caused by the lens hence the smoother HF response.

Perhaps you're familiar with the 'tissue paper' tweak often applied to Yamaha NS10 monitors Tenson? The same applies here where a couple of sheets of tissue paper are used to purposefully introduce comb-filtering to attenuate the HF output on the Yamaha's. I remember seeing an article somewhere on the web with measurements for various types and brands of tissue paper!