I'm sure your ESL9's sound great.
Haven't heard them but with the reworked woofer arrangement, low end performance must be improved compared to Theos. I use 2 subs to make up for their bass shortcomings.
Sbooster can improve in certain situations, as you've experienced. In this case, expectation bias, if anything? This hobby/obsession is supported by it, so perhaps it will help😂🤣!!
Years of living with the Theos, I still hear moments of greatness from them.
If I do stick with panels during the "big upgrade" it will be BIG panels like Soundlab.