I’m familiar with ATCs (mine pair is the ATC SCM12 Pro passive monitor). These are sealed/acoustic suspension designs with terrific bass that disappears rather abruptly below the resonance frequiency, which I believe is in the mid-40s.
As for the high pass, after trying all manner of workarounds, I ended up buying a used but perfect-functioning electronic crossover, the Marchand XM66, which has 24 dB/octave slopes (ideal for your application) and a variable crossover. I’m accustomed to setting mine anywhere from 70 Hz to 80 Hz. That means:
-- all frequencies below the crossover point go to my subwoofer (JLAudio e110);
-- and all frequencies above the crossover point go to the ATCs, or the other pair of speakers I use on my desktop, the vintage KEF 103.2s, also a sealed 2-way design
I chose this crossover frequency because the sub is so capable in that range; while the ATCs, wonderful though their bass is, are working harder the lower the frequencies go to the resonant frequencies. So I relieve them of their hardest workload.
This electronic crossover is very transparent acoustically. I really can’t distinguish between it being in-line vs not being in-line.
That’s my recommendation: an external crossover with high crossover slopes. Note that Marchand also makes electronic crossover with fixed crossovers (via internal cards you can purchase). And example of that is the XM9. The most elaborate of their crossovers, the more expensive XM44, allows you to individually select crossover cards at the frequency and slope you specify.