They always say the most important piece of safety equipment in an automobile is the nut behind the wheel. Well, the key factor that determines a dulcimer's sound is the player. Heavy strum, light strum, where to strum, type of pick... never mind fingerpicking. Five players can take the same dulcimer and make it sound entirely different. That's one reason why we'll never come to a definitive agreement about wood or anything else.
I once had the opportunity to work my way down a vendor table, playing standard McSpadden hourglasses that were identical except for the wood. I once browsed the Folkcraft showroom, where I sampled various woods and body depths (and other factors, too... the Folkcraft options can get overwhelming) (clearly I need to buy them all). I borrow and play every dulcimer I can lay my hands on, and over the years I've come to some conclusions about what I like, but someone else could do the same and come to entirely different conclusions. Different ears, different personal tastes, and different playing techniques.
If at all possible, try to attend a dulcimer festival or go to Fort Wayne and visit the Folkcraft showroom. Or go to Mountain View and visit McSpadden. Even if you don't buy a dulcimer there (good luck with that) you'll get the chance to play similar models with different woods, shapes and sizes. Barring an opportunity to try dulcimers in person, your best bet is to talk to builders, and maybe you can listen to them play some models over the phone.
Incidentally, as a former apartment dweller who likes to stay up late and hates to annoy neighbors, I've tried keeping dulcimers quiet. A softer pick, a slower strum, resting the dulcimer on a towel or some other thick fabric to keep the back and sides from resonating, using a piece of paper or a corner of an envelope as a pick, giving up on picks entirely and using my fingers, stuffing fabric into the sound holes, leaving the fabric resting on top of the dulcimer, tuning the strings down to low tension, playing near upholstered furniture, rugs and curtains that absorb sound rather than in an empty room... there are many things you can try before giving up on a dulcimer as too loud or too bright.