This has much to do with overall speed, ability to reproduce an accurate impulse response in the lower registers (ie no out-of-phase bass ports), and aligned mf/hf phase coherence by avoiding all sorts of interpolation issues between drivers at the crossover. Some less savvy mfgr's even intentionally wired the woofers out of phase with the rest and you certainly don't want that.
These are reasons speakers designed for no to minimal crossovers or simply using single drivers are more coherent for less $, however they're also usually ported so limited at building an accurate foundation to a musical event, which I believe reduces perceived coherence (unless you only listen to clarinet recordings or sthg). Certainly pistonic driver motion is a requirement, also. Flex creates phase distortion. All of this is why truly great full-range speakers are so expensive.
So we're talking about things with highly evolved crossovers and multiple drivers (yet no port). This is it, in an expensive nutshell. All this also, unfortunately, makes the speaker larger. So we have a large, expensive nutshell.
I'd recommend Evolution Acoustics. Secondarily, Magico. Thirdly Vandersteen (Treo CT and up). Possibly also an honorable mention in the budget category.. the Elac Adante (tower). I've spoken with Andrew Jones and he seems to know what he's doing.
If you're budget constrained beyond that, but have the space then i'd recommend finding some well-cared-for second-hand Dunlavy's. Freight may cost as much as the speakers, hwvr.
