Everything I read (and people I talk with) suggests that, although there are a seemingly infinite number of contributors to sound, the top wood has the single largest impact.
Let me guess: none of those you read or talked with that told you that the top wood has the "single largest impact" specified what kind of impact they were talking about, nor what precisely about the top wood was making the impact, nor for that matter, specified what was being impacted, by the soundboard.
Am I right?
The impression you must have received was that just that somehow, some kind of "good" flowed from making some kind of "good choice" about the soundboard. Kind of vague, right? The advice sounded pretty convincing (after all, the strings sit right on the soundboard, right?), but it nevertheless left you with that funny feeling that you had been given important information, but you didn't know whether you were smarter now after knowing it--or not.
The advice must not have been so enlightening, because here you are asking me to confirm or decipher it for you.
Welcome to the world of guitar lore.
If you're interested in eventually making consistently good guitars, my first advice is to a) learn to recognize lore when you hear it and then b) disregard it.
How can you distinguish what is lore? If the source is a person, ask, "how have you come to that conclusion?" He better sound very convincing to your well-developed skepticism. If it's something written, if it is couched in broad, vague generalities ("the top wood has the largest impact"), put all the red flags up. There's nothing to be learned here.
There is no "best" soundboard wood specie, so don't waste your time trying to find which one it is. There is no "best" bracing pattern, so, likewise. There is no "best" body wood, so, likewise. And so forth. So what makes the greatest "impact" ?
Precision of construction
Quality of cut and seasoning of materials
Precision of scale and saddle placement; uniformity of fretwork
Architectural choices: i.e. structural efficiency and minimal adequate structure
Closely following precedents set by cultural models
The precedent set by cultural models is, with a very few exceptions, coniferous softwoods. The precedent is vertical grain, resulting from the plates being sawn from a split billet. The demands of woodworking excellence is that the workpiece be scrupulously well-seasoned. There is no precedent for species.