Early on, I restrung many pianos without adjusting crown in them. I was always disappointed in those results. Once I learned the recrowning techniques, I was blown away at how much difference there was after recrowning. The difference is an old piano often sounds "thunky" in the low tenor section above the break. I liken the tone to that of banging on radiator pipes instead of strings. Baby grands are especially bad about this. I listen to the resonance of each board before crowning, and again after crowning, and there is a huge difference.
I am talking about a piano case and soundboard alone. All strings and plate are in another room. A well crowned soundboard will ring like a tympani with not a string in sight. Also you can listen to the tuning of the board alone by whacking it in various places with the heel of your hand. Near the bass bridge the ring is lower pitched and near the treble bridge the ringing is higher pitched. A crowned board will ring a second or two. An uncrowned board will go thunk like hitting your dining table with a fist.
Yes the crown, when returned, corrects most downbearing problems the piano used to have. This noticeably affects the ability of the piano to stay in tune. Crown is originally put into the board by shaping the ribs before they are glued on, AND the board is kept in a completely dry room. The ribs are glued on and the crown is there.
While still dry the board is glued into the piano rim. The heavier the rim the less likely it is to let the crown flatten. Then the finished unit is brought out into the normal moist air. The board is unable to spread out because it is glued into the rim, now. The soundboard takes on moisture in the air and grows wider across grain. The only way the extra wood can expand is up. The soundboard takes on moisture so fast that it is almost possible to see the crown rise as you watch. You cannot depend only on this extra moisture driven crown, since you do not know what climate the piano will be put into in the future.
This is the problem with a certain soundboard (so called) "expert" in Ohio. I have seen 4 of his pianos into which he put new boards. Every one of them was great when it arrived from Ohio, but within 6 months the treble no longer had any sustaining ability, which was the whole reason he got the piano in the first place. When the moisture from the Ohio river finally left the board through the finish, there was no crown left. He bragged to me that he put no crown into the ribs, but relied on moisture alone to crown his work. One of his pianos was in my showroom. It had no power in the top end.
Doug L. Bullock, copyright 1999