solobone22 - isn't it lovely when you dial in your speakers to the room?
I've never used any sub except Thiel's with the matching Passive XO. I learned a few things. The 'normal' input engages the room boundary circuitry, whereas (I think) the LFE does not. That boundary interface is magic. The bottom end of the 2 series is 4th order reflex, which is optimal for matching a 4th order low-pass / high cut on the sub. The 2.7's -3dB point is stated as 23Hz (can that be true?).
I'm speculating here, but since there is so little musical energy below 30Hz, I wonder how it might work with the 2.7 running free and the REL selected for as low as you can, even if above the speaker low limit.
I do hope that folks with greater sub knowledge and experience check in. You can use REW to graph the actual system output from 0 Hz. So at least you won't get lost.