Speakers replace or improve?

You are in a difficult situation. Pushing your speakers to produce more bass is going to increase distortion and rob you of headroom. Your main problem is where you live. If you want to improve your bass you will require a new living situation. Having said that many digital preamp processors have very functional EQ capabilities. Examples are Trinnov, DEQX, Anthem and MiniDSP.

If this is the case, the issue is with your turntable and or the phono amplification. Buy a $149 Schiit Mani 2 and bypass the integrated phono input. Ensure the turntable is mounted on a solid surface. Remove lid if possible when playing.

It is also connected to the same amp and the speakers. The bass is much better when I play compacts

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).

Oh, fuuuuk me, it’s an April Fools Day Joke!

I think you are wrong about Loudness control. Below is that the manual says about Loudness:

"Making it easier to hear the high- and low-frequency ranges even at low volume (LOUDNESS)

Enjoy natural sound even at low volume by lowering the mid-range sound level and compensating for the human ears’ loss of sensitivity to high- and low-frequency ranges at low volume.