Dullness rhymes with:
awfulness, coolness, fickleness, forcefulness, foulness, fullness, gentleness, harmfulness, idleness, illness, peacefulness, playfulness, reasonableness, resourcefulness, smallness, stillness, truthfulness, usefulness, wastefulness, wellness, wistfulness, wonderfulness
Dullness sounds like:
d'alene's, dalliance, dalonzo, dateline's, datelines, dawdling, daylong, deadlines, deadliness, dealing, dealings, delancey, delancy, delange, delaying, delling, dellums, delmas, delmonaco, delmonico, delong, detailing, dettling, dialing, dilemmas, dilling, dillon's, dolence, doling, dolinski, dolinsky, dollens, dollins, dooling, dowling, dueling, duling, dulling, dulmage, dulong, dwelling, dwellings, dylan's, dylans