You need several measurement spots to get good results, but expect to lose some “life” and “dynamic” when room correcting. Almost everyone feels that way, and the reason is that some sub-100 frequencies will be lowered, which gives the indication that you lost something. In fact, you will probably gain dynamic, but lose some volume, and it takes a little time before you realize how much of an improvement it is.
Btw, feel free to follow my REW guide here: A guide how to do room correction and use it in Roon and then you can compare that with what Thierry gives you. Thats what I did, and the HAF filters provided a SQ improvement at least for me.
If what you get from Thierry sounds worse, then give him a chance to fix it 