Many thanks for providing the links to your test galleries.
Using slightly larger images in your gallery might help.
I notice that your current images are 900px x 600px and, when using imageScaleMode="SCALE_DOWN", they will not be scaled up to fill the gallery's image area in a large viewport.
Using larger images should help to ensure that the images are displayed as large as possible within the gallery's image area.
Give it a try in another test gallery (perhaps using image dimensions of 1152px x 768px) and see how you get on viewing it on your iPad Air.
As long as the images are larger than the gallery's image area (which, in a responsive gallery, will depend on the size of the user's browser window), Juicebox will dynamically scale them down slightly to fit within the available space (and they will be displayed as large as they can be within the given area).
... the image is a little enlarged with a little less quality.
If you still see some degradation of image quality (even using larger images in your gallery), they try viewing the gallery in a different browser (Mobile Chrome, Mobile Firefox, Mobile Safari) to see if this makes a difference. Different browsers will likely have different resizing algorithms and you might find that certain browsers give more visually pleasing results than others when images are not displayed at their actual size. (Unfortunately, there is nothing that Juicebox can do to change the way that a browser displays an image at a specific size but it might be worth trying to at least eliminate browser differences as a possible cause.)
Hopefully this will help.