While I value a lot of things about a speaker, perhaps at the top of my list is tonal balance and dynamic scale. Without proper tone, a speaker is fatally flawed. But a speaker must also get the dynamic scale of instruments, singers, and bands right....otherwise it just sounds like a stereo. This occurs at a macro level like when your favorite rock or blues band kicks it in high gear, but also at the micro level, so that a cymbal crash sounds right and decays properly, or a wicked trumpet solo is played. More than anything tone and dynamic scale is what defines the sounds we hear, what the essence of music sounds like.
I also want a speaker to image well, to create that illusion of a performance. Now it does not need to be bigger than life, but to create a nice layered and stable image with the speaker disappearing. Also the mids have to be spot on, with a naturalness that conveys what a voice or instrument sounds natural.
Bass isn't a huge deal for me, but I do like it. As long as it is fast and tight i don't care if it does not reach the lowest of low. But I don't want it to stick out like a sore thumb. Having a cohesive sound from top to bottom is a goal for sure.
Finally a speaker should be efficient enough to be used with low powered amps. Not only is this a characteristic that aids that ability to render dynamic scale, it also allows for amplification that uses fewer parts, shorter signal paths, and a more direct design. I am not a fan of 200 watt plus amps into 85db efficient speakers. I prefer 8 wpc into 96 dB+ speakers.
And this usually means horns. I love horns and bass bins. I cannot afford a pair right now, so i got the next best thing. a co-axial 10" speaker in an open baffle arrangement and a pair of 10" bass drivers on each side. But I am plotting for horns, they are going to be my next big purchase. It may take a few years but i will get there. The horns could be made of wood, or an automotive painted fiberglass, or even paper mache. Yes there is someone building a very intriguing paper mache horn these days.
Regards
Mister Pig
Thought I'd give this thread a courtesy bump.
Mister Pig is after much of what I'm after with my main system. When he speaks of tonal balance, I assume he means timbre. This is often a characteristic of the better large-cab efficients from the '50's and '60's. A piano sounds like a piano, a guitar like a guitar, etc. When a dog barks (like if you're watching a movie or listening to "Pet Sounds"), it sounds like a dog, and enough to get the neighbor's dog to bark (Not that I ever made a practice of that.

Just stating an example). And I agree about dynamic scale as well. Once you've had that and gotten used to it, there's no substitute. I also like a laid-back speaker. The old ported, tube-era speakers with simple crossovers seem to handle added EQ quite-well, and my tube rig is very-good at adding a little extra bass when I want it (Bass-LIFT they called it), OR some extra highs. I don't touch the mids. They're already perfect. If I feel like tweaking the mids, I do it at the speaker.
But reading through this thread, you really get a sense of how much individual tastes and expectations vary. I love big speakers, but my W90's are possibly the biggest speaker I can get away with in a 12'x14' space without the sound getting shouty or too-in-your-face, which I really don't like. I learned fairly-recently that low bass needs distance. The farther back I sit, the deeper the bass digs. I really need to push my speakers back a bit. I just haven't yet because it'll take some effort, including the task of measuring, moving them a bit, measuring again, moving them, remeasuring, moving, re-re-measuring, etc. It would be simple, but I have them on heavy homemade stands, plus they're heavy speakers. I don't know, maybe I'm just lazy. Yeah, that must be it.
I want real-sounding, the artists are in the room, midrange.
The Duplexes do that for me.
That's what W90's and the other large-sized Wharfedales do best, thankfully. I would bet that Crestwood23's current main setup (the one he plays most) rivals the best home setups in that regard. It must really be something. I've heard stories. I'll get there myself soon enough. Just need to save enough cash and keep a sharp eye out there for the pieces I need to complete the puzzle. Can't wait. This should be the one, the amp/pre/speakers combo that should do it all for me, and at that point, you can throw all the tube amp distortion specs stories you want at me, and it won't matter a bit.
Actually, it already doesn't matter. I love my gear. It's the ROOM that I hate, and the electrical outlets and wiring here. But I'm already working on that. I shuffled some things around to lighten the load on the living room circuit (which in itself made a substantial difference), plus I have a high-quality UPS on it's way that should give my system a real boost and keep the voltage nice and steady. It will also filter the electrical and get rid of all the calamari in these lines (Ever seen "The Matrix"? The "squiddies" are all the noise and distortion compromising the signal and my system's response). I should have it by the middle of next week, if the tracking estimate is right.
One thing I require of my gear is that it has to be able to funk with the best of 'em. It's gotta' make me wanna' get up and shake my groove thang. Engaging musicality is extremely-important to me. The bass must be clean and coherent enough to properly-maintain the groove, so there can't be any overhang. No one-note bass. I have to be able to feel that connection between the bass and drums as far as the groove is concerned. And since I love funk so much, my system has got to be able to properly-reproduce instruments like the clavinet, Hammond B3 organ, horn instruments like the baritone sax, string section instruments, the Rhodes electric piano, and all those old-school electric guitar sounds. Even handclaps. I really got spoiled on that stuff with my current setup, so anything less just won't do, at least not with my main system.