The question was about the small room, where the max seating distance is approx 6 ft or so, meaning the seats are totally against the back wall. Now, as we know, smaller the room, bigger is the room gain. Meaning, lower frequencies will get a lot of boost. Top to that, the seating is right at the back wall, where the bass will be even louder. Adding any sub, will make the bass very louder after the room gain is added. If one plans to use absorption, it has to be placed on the back wall, the thickness needed of the traps will be more than a feet and it will even make the room more smaller.
If one uses a bookshelf, having output of -3db at 80 hz, that means, it will still have some output below that frequency. Its just that it is less than -3db average specified output. Couple that output with room gain and there could be some usable low frequency output, good enough for that room. Again, each room will respond differently, so there is no guarantee that it will work or the outcome will be desirable. That will depend upon the actual measurement of room. But it will be better than having a sub though. Sub will definitely make the low frequency response of such a small room boomy.
Wonderfully explained:clapping:; You definitely have a lot of technical knowledge to explain why sound sounds the way it sounds. But I'm really perplexed as to why you implied that I was spreading misinformation here when I essentially said the same thing (that a bookshelf would sound like a sub in a small room)