I have both a Summit 2-channel system and a 5-channel all Final system with a Descent crossing over at 80 Hz. Much to my suprise the flat Finals exhibit less of a sweet spot than the curved Summits. One gets just the same projection of sound over the full width of a large couch.
The Summits which have a much wider sweet spot than any ML I have owned in almost two decades, still change character even when moving a person's width over on the couch. It does not however lose it's "just here and nowhere else" pattern that even my Prodigys exhibited in spades.
When you look at the Final's membrane you can see clearly that it's geometry is not uniform over the entire area. It is divided into three distinct areas varying in size. I was told that this gives them their more uniform dispersion pattern, I have no idea of the science behind that though.
The Finals have been moved into my music system on occasion, but they will never take the place of my Summits. They simply are not extended enough in the high end and don't do violins and soprano voices with equal ability.
In my 5-channel soundtracks this reticence may work to their advantage. So many soudtracks are overly hot with "spitty" voices and my HT system is all solid state, whereas in my music system there is not a transistor to be found.
I have always thought that the five voices in HT can be rather distinct in themselves. Different boxes and drivers even from the same brands don't always speak the same way. A 5-channel all Final (no crossovers whatsoever!) can be astonishingly coherent. Go for it if you can, you will need a really fast sub as well though.
