Heard the B&W N804d3s ...

I had a similar sized room with the much lesser B&W DM640’s. They worked well with a high current amp (Classe CA-150) but were very lean with other amps I tried. They were too hot for nearfield though and I upgraded to Vandersteen 3A (now Sigs) to cure that and loaded the room with treatments. It sounded OK at best. I did demo a sub and found sub 30 Hz could not be heard in the room but down the hall it was plain as day. Eventually I moved the system to a 3x larger room and placed the speakers at each end of the couch I sit on. Ideal? No way. But the speakers opened up and breathed life into the music in that larger space. There’s imaging but no soundstage so in a way it’s like headphones. I’m fine with that. The quality of the music makes the system sound so much better than the loss of soundstage. So here’s my take. The room is too small for full range speakers. A sub may help but not below 30 Hz. Maybe try a bigger room even if speaker placement is compromised. You could find out like I did that you're missing a lot more than you think.

@wlutke ... Thanks for the suggestion. I’m kind of stuck in this converted bedroom as all other available space is taken up. So, I will try the sub since the lowest FR of the N804s is 45Hz +/- 2dB, which is a bit lean even when compared to some monitors; the sub may fill in the deficient area. Since I listen to 2-channel only, and don’t do HT, FR from the sub at even 30Hz should do the trick. If that doesn’t work, I may just keep the sub and sell the N804s for a monitor speaker.

Timing is a key factor to the level of acoustic problems you can get in a room. When a loudspeaker has a faster response you will have less acoustic limitations.

B&W is not the fastest and their older series are even rather slow compared to most speakers today. A subwoofer can be a good option, but again you still need a sibwoofer with a fast response. Choose always for a  sealed subwoofer.

Even when a subwoofer has it's  own roomacoustic measurment system, it is still not in phase with the front loudspeaker.

You need a system what is able to measure both speakees and subwoofer togheter. In that situation you will always be able to outperfrom any loudspeaker system for the same money without a subwoofer.

Yep. Ordered a Rhythmik L12, which is a sealed unit. Hope it does the trick.

The Rhythmik L12 is a subwoofer with a driver made of paper. Paper is an outdated material these days. Beside this paper has it’s own kind of sound. It is not the most natural soud.

You should have spend more money on a subwoofer if I were you. I would buy a subwoofer with a ceramic/aluminium coating.