Which speakers will fill 5,000 cubic ft coherently

they easily fill my room ;) The soundstage is as wide and tall as appropriate for the music. I once had some Audiostatic electrostatics and the soundstage was TOO tall - singers were like 10' tall, heheh...

I'd direct you attention back to the Selah line arrays. I have a pair of their custom Incredarrays (similar to the Alexandrite) and they will definitely fill your room, with great dynamics and proper room height. The height thing bothered me with many speakers I owned until I got the Selahs. There's a thread over at Audio Circle called 'why line arrays' that you might find interesting. One post from that thread:

"1. They fill a room more evenly with sound, not too loud up close or too quiet at a distance. This is because, as mentioned elsewhere, the SPL from a line array decreases by 3db for each doubling of distance vs a regular speaker that decreases by 6db. This is a huge difference.
2. They have a sweet spot that is orders of magnitude larger, a direct result of #1.
3. The nature of how sound disperses from a line array leads to better in-room behavior. Not only is floor and ceiling bounce virtually eliminated, but that also eliminates the floor/ceiling room mode from having an influence.
4. They sound "bigger"."

I wouldn't be overly concerned about a SS amp driving the sub frequencies. And even with music with minimal low bass, the lower extension that subs provide will add greatly to the 'realism' of the performance.

Pick your poison. I would go for a wide range single driver midrange. Look at the classics like Ls3-5a, original Quad, Spica TC-50, original Advent, etc. All were as simple as possible. K.I.S.S.
Time has taught us (some of us) that complexity brings with it complication.......and problems.

I'm impressed with what I've read about SP Tech. I spoke to Bob and he does not feel the location (near the front wall) or size of the room, or the fact that I have a tube OTL amp are impediments. (I know, I know, 8 ohms is better)- but I still haven't found the ideal speaker in theory, and SP Tech is getting raves. Comments?

Thinking over your problem again I think I'd be taking a look at reflections from the floor under your system.
Your high ceiling may be changing what might normally be balanced by an 8 ft ceiling.
Could you advise your flooring around your system & speakers?