But in any case, I may have overemphasized the importance of the dimension ratio. With low frequencies standing waves usually will not form. I guess I just got into the habit because of building many mid/treble boxes...

But either way, If your box is not as strong as the rock of gibralter, your output will not be maximized. Stronger is always better, for ANY speaker (well, not tweeters of course :-). The less sound that comes out of your box through vibrations and the like, the better your system will be.