[mflaming]

Do you think it would be okay to alter the height of the XLS cabinet? This is just for cosmetic reasons. They seem a little squatty to me.  I’d like to add about an inch and a half to the height.

[Hobbsmeerkat]

The height is about the only thing you can modify without affecting the response. so adding an inch or two to that model isn't a problem,
(so long as nothing else changes)

[E-Zee]

I'll give a second vote, in agreement with Hobbs.  Keep the width the same. Keep the driver spacing the same.  Add some height. 

I've experienced increased height / decreased depth modifications on several designs without impact.  In a few cases that height adjustment was to accommodate a front slot port instead of a traditional rear port.   The baffle width,  and driver spacing should remain the same and your adding a little to height won't have detrimental effect. 

[Danny Richie]

Only add the height below the woofer though. Don't change the tweeters height to the top of the cabinet.