Is this a joke?
Keep in mind, bass is only part/not a lot of the music you are playing. To cut higher frequencies out all the time is not the way to solve it.
"The treble and mids are ok but the bass is very weak."
(Polk's two 8" woofers and a front port, in a small room should sound good)
"To make the speakers to produce some bass I reduce treble"
(bad!), (the speakers are producing all the bass they can, cutting treble from everything, all the time will not sound good)
"reduce the Loudness setting from Flat to -30db"
(bad!) ('loudness' is for low volume listening. it boosts bass, progressively as you lower the volume) (cutting the effect of loudness is going opposite to what you want)
"and turn volume to very high. But I live in a townhouse and can’t do that most of the time".
('loudness' is what you want, especially at low volume).
(normally at normal/loud volumes, the 'loudness' filter should be OFF to prevent too much bass).
(for you, a self-described bass freak: you could try leaving 'loudness' on when listening loud, leaving every other frequency alone, just boosting bass))
"If possible I would like to increase the bass. I know that this question was asked many times and there is no universal solution. My home stereo is located in the small living room which practically doesn’t have a back wall because there is a big opening between the living room and the dining room.
(are there partial walls at the sides, i.e. corners?)
(if not, you could build short walls, say 3' long behind each speaker, put the speakers close to both side and rear wall).
I also have the Polk Audio PSW108 subwoofer but I don’t use it because there is no room for the sub in my living room."
(you need to figure out how to use a PAIR of subs, self-powered, front firing, located adjacent to the Polks).