If not using the internal woofer of the ET’s I’m wondering if OB subs can handle (well) covering from 180Hz on down?
You’re thoughts on using a pair of F12’s to just bring in the bottom end of the ET’s we’re my original thought. If a pair of triples would cover from 180Hz down then this sounds like the best way to go.
A quad in OB (two per side) worked just fine when I did it (take the ET woofers out of the chain completely). IF I had triples at the time, I would have used them.
In my case, I ended up leaving the ET woofers in and using the sealed subs because the amp I had at the time didn't like playing as nicely with the load the mid/tweeter circuit of the ETs alone versus when the woofer was back in the circuit, and I was also doing double duty with the speakers for home theater in a medium-smaller room, so I needed the subs to pressurize the room for action movies occasionally. How the room is loaded (or not) aside, for music, and playing that far up (200Hz-ish) rather than crossing down at 80Hz-ish, the OB subs were clearly better, and did change the lower midrange of the speaker a bit in character; I'm not even going to try to articulate much past that because it was a very long time ago and I can't reliably remember enough to make a good showing of describing it.
I'm pretty confident that not only will you be very happy with the OB triples, I wouldn't be afraid to put money down that the subs will outlast your speakers as you continue on in the hobby if you change your system later over time; they really are that good.
One more thing - when you put the OB subs in place, if you can, high pass the signal going to the mid/tweeter amps so that they don't get any of the low frequency signal (I crossed them over with different points between 60Hz and 120Hz when I was messing around tuning the system I had). Even with only the mid/tweeter circuit running where the mid driver is only playing down to 180Hz, offloading the lows from the mid/tweet section really made a difference for the better when I did it; the change wasn't night and day, but wasn't subtle either.