Ride comfort (on a rigid bike) is primarily a function of the tyres (size/inflation/construction). A bike with cheap 23c tyres at 150psi will rattle the fillings out of your teeth. The same bike with decent 25c or 28c tyres at e.g. 50-70psi (depending on rider weight) can float along with a magic carpet like ride.
To clarify - vibration that it best dulls is 'road buzz' which you can really get on a alu bike, esp with high pressure tires. A carbon seatpost and seat can be as little as £50 for both - I got my seat post from wiggle and seat from china - happy with both - and recommend to all my friends.
Yes this will have little impact on 'bumps' - though the saddle def soaks some of them up.
I posted this to refute that 'only' tires have an impact on ride - frame, wheels, seat post, saddle, bars and bar padding, shorts etc can all have a positive impact on ride - and allow you to ride at a high tire pressure and get all the benefit of lower rolling resistance.