Depending on the sub! One sub can sound smooth in the room , this is best dealt with if one takes the context as a whole dedicated room or existing seating position, sub and speaker location and treatments and digital RC. My room falls into the existing category and I can only speak to that end. the steps i've taken to get accurate bass at the listening position takes time and error as I depend on my ears and the nod from the wife to make final setup decisions
My room is almost a perfect cube with a large brick fireplace and two large openings on the right rear and right that leads to the kitchen, dimensions are 20x21x8 , I have the 3 corners bass trapped (2) 24x24 4 inch open back bass traps behind the mains and an GIK 24x48 7 inch monster trap in the rear of the room and a 48x24 4 inch behind the main listening position 6ft away, my sub is a KK DXD12012 in the front right corner 2 inches from the side wall and 6 inches from the back wall as per ken's instructions, I also Audyssey XT32 to dial in the sound along with (2) 48x24 2 inch panels to deal with first reflection points and the front wall behind the display and mains have (3) 24x24 2 inch panels. the end result is smooth bass across the 5 seats and a room favoring 50hz bump is tamed and lower frequencies are as pronounced as the upper bass not to mention seamless integration with the mains center and surrounds! no matter where an effect is placed within the sound field the DXD12012 is there to act as an independent sub for that speaker and even the phantom pans between the speakers
my two channel is seamless as well ( and also my first priority) So yes depending on the subs ability to produce clean bass upper,mid or lower it can be done with one in the room and get high spouse approval at the same time! not boomy just great bass!