Actually, the LX would be a very significant upgrade in performance from your CMT-340SE. They do require more power than the 340's to reach the same volume level, but at your listening levels and at your listening distance (and room size) - this is of absolutely no concern.
In addition, the LX actually do a lot better at low volume listening compared to the Sierra-2 and dialog intelligibility (both on-axis and off-axis) is better than the Sierra-2.
The bass response of the LX crushes the 340's and Sierrra-2 and while being quite a bit smaller than CMT-340, the LX actually sounds much larger than the 340's while also presenting a significantly more spacious soundstage.
Huge performance upgrade in every possible performance spec
Hope this helps and please feel free to reach out directly.