I just put a movie on Netflix where the original format was 5.1
I Set 3 speakers to avr configuration . So the dialogues were set to the center, using the original format but for only 3 speakers. After that I just switched to stereo from surround mode settings and the dialogue seemed to sound better.
After that I just moved the second pair to surround duties set to 4.1 , using the original format but for 4 see peakers now.the dialogues seemed to sound better than 3.1
Seems very strange. I will try to reach out to Arendal . I am curious to hear their thoughts on this.
I've tried a couple of times using bookshelf speakers as center as well... and you are right, it always sounds odd, unsatisfactory or just different for some reason.
Could it be the tv just behind and above? Or could be power or impedance related as dedicated MTM center speakers are usually more sensitive? I would try to experiment with LR Toe in, speaker to wall and listener distance or tilting.
Also possible that you just have to turn up the playback volume or turn off/on settings like Dyn EQ and loudness mgmt. Great if we could see some measurements to compare. Have fun.