To the OP, I've noticed you have posted this question on multiple forums, with "no bites"
Probably because no one has used your particular 2 way combination. You need to determine and post what frequency response you want to roll the Heils down to, and the (preferably) 1/4 wave distance you can allow to the driver underneath (or above). More than likely, for a 2 way, I would choose a 12" driver. In fact that is what the OEM has on their web site (but with a passive radiator also).
Realistically, you should use a mid range or wide band between the two. I have read the Heils will distort if pushed too low. Unless you add horns to them or double/triple stack em.
https://community.klipsch.com/index.php?/topic/188177-double-stack-ess-amt-1-with-wings-possible-kit-for-heritage/If cash is tight (I can relate), for a cheap midrange, I have seen this driver used with the Heils, in a 3 way:
https://www.madisoundspeakerstore.com/special-sale/eminence-beta-8-8-pro-mid-bass-8-ohm/I actually picked up 4 of these, for the future.
Not to diss this place, but go to the diyaudio forum (where you already have been) and use their search engine. You will get lots of hits regarding the ESS Heil AMT-1s, I get the impression many folks use them there... Full disclosure, I am building a 3 way with Heils, but not "there yet".
