Filter the sub, not the 'stat!
Sorry, physics (and THD measurements) tell me that it's a great idea to keep drivers operating in their ideal passband.
If the ESL is operating in its resonant frequency, the THD shoots way up.
Personally, I've been working hard (and spending dough) to keep the rig at sub 1% THD at 105dB SPL. Letting a panel play too low kills that.