A few questions -
What's the budget?
Sealed, ported or?
Maximum size?
Ask any speaker designer to suggest their perfect speaker, and they'd all give a different answer. For me, with every design, I always learn something new and when thinking about the next design, I try to improve on the worst aspects but keep the good bits.
Perfect phase coherence
no colouration
easily driven
Perfect phase coherence - The problem here is, without a hell of a lot of attention to detail when it comes to measurements, and processing those measurements correctly for the crossover design software. You won't have perfect phase coherence, even though the software is telling you, you have. I've lost count of the number of times that someone uploads a graph showing the phase tracking beautifully throughout the crossover, and then we find they forgot to include offsets in their simulation! With an even order crossover, you can reverse the polarity of one of the drivers, and take a measurement to make sure you have a good 'null', but that doesn't mean the phase is as good as it could be.
No colouration - Obviously that's impossible, but your average speaker is such an imperfect thing, that there's plenty that can be done to lower colouration. Most speakers are rectangular boxes, and this is bad because as the sound diffracts off the sharp edges, it causes peaks and dips in the frequency response. It's argued that this is what causes a narrowing of the soundstage.
The ideal speaker enclosure shouldn't sing along with the music, IMO. The stiffer the cabinet, the higher in frequency the cabinet resonates, and this often ends up where your ears are most sensitive. Thin wall cabinets with no bracing, don't sound as tight in the bass. I think I've found a good solution with stiff, reinforced foam, but although it's easy to work with, it's not easy to cut accurately or achieve a nice finish.
Easily driven - Ported speakers are usually easier to drive than sealed, but I know there are a lot of fans of sealed on pinkfish. Ported done properly beats sealed IMO, but it's not easy to get right, and that's why sealed betters ported the large majority of the time.
I find higher order crossovers sound too forward, but low order are difficult to get right when crossing over at higher frequencies. The drivers need their relative acoustic centres to be somewhere near each to achieve good phase alignment, either by having a step in the baffle (causes diffraction problems), using a waveguide and small midrange or an angled baffle. The other option is a dome mid because the acoustic centre will be almost, if not completely identical, to a typical dome tweeter. You also need drivers with a smooth, extended frequency response, and because of the shallow slope you don't want a breakup peak that you get from your typical hard cone.
For a dome mid I quite fancy the
Scanspeak D7608/920010 as used in the
Alchris Audio AR-66 or the new
Bliesma M74S
There are plenty of nice tweeters out there. I personally try to avoid ferrofluid, because removing it in experiments made a surprising improvement - This is a nice tweeter for the money (
Scan D2604), and looks like it could be the same one used in the
Proac DB1
There are lots of nice high efficiency 10" to 12" pro audio drivers out there. It just depends on what size driver you'd want, or if you'd rather go sealed, Peerless and Scanspeak make suitable drivers.
