Speakers replace or improve?

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 are probably sitting right on top of a couple of strong modal nulls (the widthwise primary if you sit right in the widthwise middle...and it is probably coupled with a strong lengthwise, a heighwise, etc). In such scenarios, you will have very flaccid bass, no matter what speaker or amp you get (might as well sell everything and start dancing to imaginary music).

A subwoofer is also a modal treatment device, if placed and set up correctly. If you only have a single sub....try placing it nearfield around your couch. Tell wife that it's a stand for your beer to improve WAF.

Oh...and in your case, it sounds like you may also want to move the broken record player's location to ---= trash can. 

fletcher munson 'loudness' curve(s)

we hear less bass and less highs as the volume lowers.

Fletcher Munson Filters (named 'Loudness' (a stupid name, it should be called 'Low Volume Boost.

As you lower volume from your normal listening level: 'Loudness' PROGRESSIVELY boots the bass, leaves the mids alone, and boosts the highs.

Your manual suggests a different solution, by progressively cutting the mids, it 'in a sense' creates the same result. This method might be good for low powered amps (that would distort trying to boost bass), and/or inefficient speakers that need more a lot of power (or a combo of both).

IN any case, you do not cut the highs.

IN any case you do not alter loudness by -30db

This is why I think this is a JOKE.

That open area is going to cause loading problems. You really need to buy a couple of good subs that will proplerly load that whole area. Which means to get good bass you need something that will really move a lot of air. Forget the cheaper subs, they are not going to cut it. That is a larger area you are trying to bass load. Do you have a friend you who will let you borrow a couple of large subs so you can try it out?

No, I don't have a friend who can let me borrow his sub. But when I turn on my sub it produces more than I need bass. I just need to learn how to use it properly because it has two knobs and two switches. 

I was trying to avoid using the sub because I can't put it in front. It sits close to the right side wall. But it looks like I need to use my subwoofer all the time. And probably buy a second sub. 

My sub allows wiring speakers to the sub. Is this a good option? Right now my speakers wired to the amp and the sub connected to the amp.