Scorpio... I purchased the Summit X spikes from ML for my legacy Summits, as Mids and Highs are very sensitive to "rake angle" (i.e. tilt) of the panel. The default angle for Summits and Expression 13A's is ~5°, but using the Summit X legs, I've adjusted mine to 1°.
Samgar2, my family/listening room is 17' x 21' with cathedral ceilings, and adjoining 18' x 14' kitchen, so total ~6400 cubic ft. You can see a pic here...
http://www.martinloganowners.com/forum/attachment.php?attachmentid=18721&d=1473094788The Expressions would likely be fine in your room, but depends on overall acoustics/furnishings. If it's a DEDICATED listening room (or Home Theater), you might still consider the 15A's if you really want larger scale sound (and appearance). Depends on budget +/- wife or SO acceptance factor!