Which speakers will fill 5,000 cubic ft coherently

You need to better specify what volume level you desire. You should also state what amp(s) you would be using.

Springbok, I now understand your desire not to switch amps, but I'm still not sure about the limitations about where you can place the loudspeaker. It's highly unlikely that a speaker of the performance you are considering will work optimally in the narrowly prescribed area you can position them. You may have boxed yourself into a coffin -- so to speak.

To take a step back for a second, you're more likely to have success if you first match the speakers to the room and then match an amplifier to the speakers. You seem to be trying to collapse these two separate steps into one. Not that it can't be done, but it greatly limits your options. In other words, why are you so tied to your amp?

If the emphasis is on coherent sound you might also consider the larger coaxial designs from Altec (Great Plains Audio), the big classic styled Tannoys or the various Phy based speakers. They are all very efficient and like tubes.

I think some people are stretching the point about tube amps not working well with sub-8 ohm loads. As a rule OTL designs prefer high impedance loads, but tube amps as a whole can work perfectly well into lower impedance speakers. If you take into account the output impedance of the amplifier you'll find that most tube amps work best with speakers that have smooth impedance responses. But that is also the case with most transistor based amps. With the proliferation of single end triode type amps, which as a group don't do low impedances, all tube amps are being unfairly tainted as incapable of driving real world speakers. Any number of Audio Research, Conrad Johnson, VTL, Manley, EAR, etc. amps thrive on driving low impedance speakers.

Atmashpere, in your opinion which is more important - the speaker's relative impedance or the smoothness of the impedance curve? For instance, would an amp rather see a speaker that rated at 16 ohms which actually varies from 12 ohms to 45 ohms, or a smooth 8 to 12 ohm load? Or is it something that can't really be expressed this simply.

BTW, your contribution to the Forums is invaluable.