And how loud do you expect this sub to go?
Subwoofer sound waves, literally, fight with each other and a user ends up with modes, reinforcement and cancellation or peaks and valleys. Two seating positions, literally four feet apart can have upwards of a 10dB difference where despite being only four feet apart, one listener perceives the sound level to be twice as loud as the other listener and for those with sensitive hearing, this is a huge variation and can will be considered an imposition.. A lot of effort needs to go into fixing this type of common subwoofer listening phenomenon and the easiest way is with multiple subwoofers. The point, are you looking for a single sub solution or is the wife going allow there to be more than one sub in her living room?
My way of characterizing subwoofers, they're the most vexing of Home Theater components and compared to other Home Theater appliances, without exaggeration, require special feeding and care to get the best out of them.
I'm not trying to confuse your question as I play one thousand questions but in your answers, readers will better be able to provide recommendations as it's easy to say, buy this sub but getting what you want is a horse of a different color for if my questions are not properly attended to, the most expensive and well respected subwoofer, can and will fall flat on it's figurative face.
Are your provided measurements exact measurement or can you go with a 18" x 18" 18" box? I know this is 1.5" wider then your provided max of 16.5".
I get the feeling you're being limited to a one subwoofer solution and this limitation, in of itself can defeat your quest for "...the best clean bass possible out of a small enclosure."