Thx ,ultra or select, matching my speakers

I think that the rating of speakers as far as THX is pretty much snake oil and would not worry about it. It will be your reciever or processor that will determine performance, the speakers only do what the head unit tells them to do.

NO. Really, the more power, the better!
People "clip" speakers, most every time, due to "underpowering" the speakers with their amp/receivers!!!
The more power/control over your speaker drivers, the better! Trust me, 23 years experience professionally.
Your THX select speakers design/config, however, is better suited to "smaller rooms", and/or room situations where you sit "closer to the speakers" in proximity vs. the room boundaries/ceiling, etc! Otherwise, you need much more attention, critically, to room acoustics.
It's all a "balance".