I've actually been there and done this with these speakers.
If you're plan is to go OB on the woofers, I would suggest bypassing the sealed woofer at the bottom of the ET's completely, because it kind of sticks out integration-wise if it is left playing. Keep in mind the planars and the subs will become a little pickier with room placement, even tweeters in versus tweeters out will make a noticeable difference, and I even played with a little tilt back on the planars with good results.
If you want to leave the ET's sealed woofer in play, I know I'm probably going to be the dissenting opinion around here but I would actually recommend a pair of sealed box servo subs, crossed a little lower than going straight from the planar driver to the subs rather than going OB; the ET's sealed woofer was barely playing, but it helped make the handshake between the planars and the subs smoother, as the planars really have a rock bottom of about 180Hz that very sharply drops off at the end where the sealed woofers built in to the speaker made a nice smooth hand off to the subs at that point, and the subs really took on a role that was foundational rather than fundamental to the overall speaker.