Re: center recommendation
If wall mounting (e.g. flush or close to flush with the wall), the HTM's, IMO, would be the best choice as they don't have a rear port. With the CBM's, you'd need to space them out from the wall a bit (at least equal to the diameter of the rear port).
As to a wall mount for the HTM's, you could use a picture hanger and picture hanging wire. Wrap the wire around the 2 screws in the threaded inserts and then hang. I had mine mounted this way for some time.
Alternately, you could use something like this. (But I'd try to find one that's rated for more than 10 lbs.)
https://www.amazon.com/VideoSecu-Uni...s%2C248&sr=8-6
I use something like in the following link for my HTM200s (as heights in an Atmos center) and my S2 surrounds.
https://www.amazon.com/TNP-Dual-Book...zcF9hdGY&psc=1
As to the S1 as center, I'd center the cabinet on the screen center line. Mainly for aesthetics. I don't think one way would be better than the other from an audibility standpoint.
* LG OLED65E6P, BenqHT2050A
* Anthem AVM90, Rogue ST100, VTV Pascal 7 ch
* Sierra Towers/Horizon (all ELX RAAL), S2EXv2 surrounds, HTM200SEx4 heights, Rythmik E15HPx2
* VPI Classic 1+ (VTA & Fatboy Gimbal), PE Eagle/RR, VAS NOVA, Soundsmith Paua, Manley Chinook, Bob's Devices SUT, SugarCube 1 mini
* Oppo 203 & 103D, EverSolo DMP-A6
* miniDSP Flex, Audiosensibility & Blue Jeans cables, Symposium & Isoacoustics, GIK
* For RDJ: Anthem MRX720, Sierra LX, Luna Duo v2 center, CXNv2, TBD