It is is a good question. But a lot of it depends on propinquity. Especially at this level of expense and exclusivity. With that in mind, I'd ask, how many different showrooms you encountered prior to buying your home theater? Do you have a lot of dealers in your Southampton area that you can demo different installations? Many homes letting you visit? Enough to run down most of the major brands? I know you mentioned a couple. Surely larger cities are likely better served, that much is certain.
In my area there are four outfits:
One doesn't list any brands they sell. Customers are supposed to call for an appointment in their home, and then get the lowdown.
Another lists about fifty brands. In the heydays of hi-fi, I don't recall any dealers that actually stocked and sold fifty brands of anything. But their Website looked promising so I clicked. In the speaker department they have the franchise for Sunfire, JBL Pro, Revel, JBL Synthesis, Origin Acoustics, Revel, PMC, Rel, Paradigm, JL Audio, Artisan/Savant. What do you suppose the odds are that this small storefront (weged between the Benjamin Moore paint store and a fabric shop) actually has a good selection of each of the brands, for audition? It is near Whole Foods. Waifu wants to go grocery shopping next week, so I'll do a drive by and check it out. I could be surprised.
Another installer inhabits a section of aluminum warehouse, next to the College Hunks Hauling Junk operation. They list Phase Technology and Oppo (!) in their brand portfolio. That sure instills confidence.
And then there's the 'installation experts' (it says here) at Best Buy.
My point being, unless you are in a pretty large city, you probably aren't going to have many options for an actual demonstration of different speakers. Even if that means going to someone's house to listen, after they get home from work and put the kids down. What you will likely get is a salesman who will first ask about your budget, and if it is over a certain threshold he will show up at your home with some brochures.
Strangely, if you go to the Grimani Systems site, they don't even list any dealers. You can learn about their CEDIA awards, and '100 Years of Experience' (which looks like the three boomer principals). One of them is Manny LaCarrubba, who certainly knows loudspeakers. Manny is responsible for the waveguide you find in some of the exceptional sounding B&O products, licensed from Sausalito Audio, which he founded. From his work alone, I'd be inclined to purchase sound unheard... if I could not find a good used set of B&O 5 at a discount.
I'd be happy to learn of ASR members home theater buying experiences, especially at the higher end, working with installers. How many installations they demoed, did they have to travel distances to investigate, did they go to anyone's house and check out an already installed system, or did they buy from a brochure, or just leave it to their contractor if it was an addition? The ASR crowd tends to be 'hands on', so I don't know if they would, as a group, be representative of folks who might drop high five or six figures on an installed set-up.
I'll admit, it's not something I can relate to very well. But I'm willing to learn how others do it.