I work on ATC-110 ASL daily and have tuned many ATC amp packs in situ on 25,50, 100s, 110 etc - it's the same pack. The do indeed use all pass filters for phase correction between drivers and it is quite effective.They are an unbelievable pain in the ass to adjust and calibrate.
If the goal is great monitoring or to make music or to save money I would strongly advise against this DIY speaker route. If the goal is to have fun and learn then have at it.
ATCs mid dome is great, as is PMCs. I've never heard the Volt dome. My next speaker project will be based around the Bliesma Silk mid dome, worth looking at. Look at the new Mum audio speakers for inspiration. Depending on where you are based they are awfully competitive price wise with DIY and won't be for long.