AParish,
The 340SE's rear bass port would not appreciate being located very close (i.e. only a few inches) from the wall. How much this positioning will noticeably change the sound is hard to say, a quick experiment comparing the two speaker positions is probably worth the trouble.
As a point of reference, my Ascend Sierra's (also having a rear bass port) are located on stands a couple of feet from the wall.
I don't know if the 340SE is designed to accommodate a bass port plug. The fine folks at Ascend would be able to answer that question and give you insight into the expected bass frequency response change. Even if the bass port plug raised the minimum frequency response a fair amount it should not hamper your overall sound production because you have the subs.
At 26 lbs each, I do not know of a mount that would get the speakers far enough from the wall and also be able to safely handle that amount of weight. Force on the mount being equal to weight times distance!
Cheers,
XEagleDriver