Speaker height vs ceiling height

I would think you want to avoid early reflections of ceiling similar to walls. Usually there is less distance to work with in regards to ceiling and floor than walls even.

I like to keep my speakers as low to the ground as possible but also with adequate elevation from floor. Having drivers as close to listening position elevation or slightly below is probably a good practice in general.

Tall line arrays might be a different story, but good ones that tall will cost a fortune.

Vertical dispersion in lines planars and stats work often in lower rooms. From what my ears say atleast.

Map,
You sure got that right! I heard Audience speakers...the 2+2 at THE in Newport a couple months ago.
5 large, but really special.....

Right in back was the line array of 16 (count 'em.....16) idential drivers. I didn't bother to ask

you know what, take advantage of the situation & use the lower height of your ceiling to make your speakers a line-array-like speaker. Line array speakers use the floor & ceiling to make them appear as an infinite baffle speaker. So, if you get a speaker with a radiation pattern that can make use of the floor & ceiling you'd have a fantastic soundstage height (& depth if you do this right). Line arrays do not have to cost a fortune (but they can). Check out VMPS speakers & those from Selah Audio:
http://www.vmpsaudio.com/
http://www.selahaudio.com/id73.html

I happen to own VMPS FF3 SRE speakers in a drop ceilng basemant and at 68in tall with the ribbon mids they work great in my 14x24 room.