On rigid bikes, tire squishiness tends to be by far the largest determining factor in how well the bike smooths out bumps. What's your bike+rider weight, and what PSI are you running those 28s at? You might be able to get big improvements by reducing tire pressure.

Wider tires can help, because they function better at higher squishinesses than narrower tires. Tires are a form of suspension, and because a wider tire is also taller, it has more "travel." So it can be run squishier without increasing the risk of bottoming out the tire on the rim. Also, the deformation of the tire as it rolls is a major contributor to rolling resistance; at high squishinesses, a wider tire will tend to lose less energy to this than a narrower tire.
But, remember that wider tires are only more comfortable because they allow you to get away with running them squishier. If you don't run a wider tire at a lower PSI, it will be just as harsh (potentially even harsher) than a narrower tire.

How the tire is constructed is also important. Lower-quality or bombproof tires will tend to be stiffer. This makes them ride harsher at a given PSI, and it also increases how much energy they lose to deformation as they roll. You can make a stiff tire ride nicely by pumping them extra-squishy, but then the rolling resistance penalty would be especially large.
If you go a bit wider with the goal of improving comfort, you should probably avoid switching to a less performance-oriented style of tire. The trick here is that performance-oriented road tires in high widths (i.e. >30mm) aren't very common. Panaracer Paselas are a decent non-super-pricey option.