Speaker height vs ceiling height

Chad,
most room mode calculators treat the ceiling as just another surface . It may absorb more energy than say.....a slab floor or sheetrock wall, but still and all, you must treat it with respect.
Construction makes a big difference.

The mitigation is the order of drivers. Tweeter at top of a TALL speaker may interact more with the ceiling while the woofer at the bottom, may interact more with the floor.

He may be able to get away with a tall line source?

I do not really qualify to address the issue, but intuitively I would certainly leave couple of feet between speakers and ceiling for dynamic speakers. Panels might tolerate less space above them, I guess. And horns...I have no idea.

My basement ceiling is about the same height as yours.
My speakers are 5' with no problems.
Don't think I would try anything taller in a speaker, but I could be wrong.
BTW - Which 6' speakers were you thinking of?

I really do not believe tall speakers are an issue. I have owned 8' tall electrostatic speakers and 6' tall dynamic speakers without any problems. Most of the really tall dynamic speakers place their drivers in a D'Appolito configuration, so the midrange and tweeters are not close to the ceiling anyway. Having the speaker near room bounderies (floor and ceiling) supports bass response and I seldom hear anyone complain about too much bass.

Yes, there can be too much bass, easily. In smaller room.