Here's what I tell new clients when they commit to a major restoration: Bass players generally like a bottom-heavy sound. When their bass is falling apart, the different pieces are not vibrating as a unit. Seam openings and cracks are spilling air where they should not be. Braces are loose and flapping in the breeze. So the overtones tend to be muffled, and the bass sounds "dark". When a major restoration is done (and done well), the bass' parts will vibrate as a unit, all the seams and cracks will be tight, and the overtones will be screaming. Because of the way we hear, those overtones (highs) will tend to capture our attention. Now the bass does not sound so "dark" anymore. It sounds sort of "bright" or "raw". In reality, though, we have not lost any bottom (or very little), but we have gained overtones, or treble. These overtones are what gives the instrument definition, carrying power and tonal color. Over time, the entire instrument will relax from the torture of surgery, and the sound will become more like what the player was used to. But it will never be exactly the same; usually it will be somewhat "better", if "better" means enhanced clarity, response, and breadth of tone.
Now to the contrary, if the restorer has a heavy hand and uses too much wood, the wrong wood, or the wrong types of glue, or puts things like the bass bar in the wrong place, just expect the bass to sound bad.