What's the weakest link in my stereo set up? I live in a townhome, so limited on space.

The first thing I’d do is add the JN-P200 or JN-P500 stylus to your MP150. The 200 adds the benefit of the boron cantilever. The 500 goes a step farther with boron cantilever, and a fine line stylus. Both are great upgrades over the 150. Amazon shows them @ $156 and $252 currently....almost a no brainer, but it is subjective.  (I added the JN-P500 to my MP150, and love it.  The friction fit is fine, and you can leave the screw off.  I added a few grams of mass to the headshell to offset cartridge weight differences)

You already have a very well regarded TT, but isolation pucks or platform can help optimize things.

An active sub (or 2) is an easy and affordable improvement as well.

How are your cables and interconnects?

Many here will ask questions similar to these? What do you think you are lacking? Dynamics, bass, soundstage etc, etc? What kind of music do you listen to? Would your speakers be close to a wall or can you have them out in the room? Stands? Are you limited by WAF? (Wife acceptance factor) in terms of aesthetics. I agree you have a good enough turntable and upgrading the cartridge could help. BUT what are you looking for, specifically? And of course, what is your budget? After all, many wives care about that too.

The room will have more effect on sound than the gear. If there's anything you can do in that regard that your spouse would approve of - and some sound treatments can still be very attractive aesthetically - Makes a big difference whether you're on hardwood floors or wall-to-wall carpeting. Makes a big difference if you have plush furniture and stuff on the walls or if your furniture is minimalistic with the walls being unbroken flat planes, to help control sound bouncing around all over. 

Can the JN-P500 fit onto the body of the MP-150? If so, that may be an extremely economical way to get some upgrades without swapping out audio.

My TT already has isolation feet on it (put on by KAB), so I think we are good on that side.

Floors are solid hardwood. No carpet. I can possibly add some sound dampening to the walls, but I have no idea about placement.

I do have the turntable turned to face the width of the townhouse (i.e. facing the wall that is a shorter distance). If I am able to convince the wife to turn the turntable and the whole set up to be facing the further walls (i.e. go the length of the townhome instead of the width), would that be an improvement? i.e. more space means the sound doesn't immediately bounce off a wall.

Cables on the turntable are what KAB put on when they upgraded the table about five years ago. They are better than stock, but I honestly have no idea how to find out the "brand" and "make".

The power cord goes to an external speed controller that KAB also gave me.

I agree best bang for buck would be stylus / cart. 

Agree with larsman. There are very attractive wall treatments available today.

I don't know how good your phono stage is, so I would probably upgrade to Nagaoka MP500 cartridge first. Or perhaps the speakers, but you say you like them.

@vrikkian Can the JN-P500 fit onto the body of the MP-150? If so, that may be an extremely economical way to get some upgrades without swapping out audio.

It certainly can.  It's a perfect friction fit.  As stated, I did add some mass to the headshell to compensate for differences in the cart weight between the MP150 and MP500, but am not sure it's even necessary.  It's subjective, but the results sound sublime to me.  

Townhouses and stereos sometimes don't mix well. Living too close to other people causes a lack of volume and dynamics. 

Sounds like getting a new cart is an easy sell. I’ll go with the new needle first.

Next, since I like my Focals, I’ll see if there are better bookshelves out there than the ones I have now that have been discontinued for some time.

Subwoofer all day long. You are missing a ton of bass. Take some of load off the 906 and let a subwoofer do the heavy lifting. Buy the subwoofer with the best specs in your budget which is likely the SVS SB-2000 Pro.

I think you know who the weakest link in your set up is. For your wife's needs, your system is perfect.

Enjoy the music.if your cart is used alot you may want to change.the turntable is good been around for decades and proven.focal are great speakers even the older models.these days you can dsp your system and room.have fun keep the spouse happy you know happy wife happy life enjoy the experiment.see if there is an audio club in your town to swap equipment.crutchfield gives 60 days and upscale audio may as well on speakers.