New TT or new Cart? Technics content

The key is the room size and the right placement of two subs if needed to fight the room modes, just another sub will not solve the problem in a small room.

For a small room even one sub is a big problem. It’s impossible to solve the bass issues in a small room, from what I’ve learned the 8 ft size of the ceiling (typical for North America) is terrible height for a ceiling in a listening room. In a small room it's better to use a bookshelf speakers or near field monitors without sub. 

My room is big with a high ceiling and I don’t use any subs. However, I want to study more about room acoustics, sadly we don’t have an experts on audiogon, room acoustics (and room treatment) it’s very interesting topic.

It’s a very large room 15x30x9 (15 across the front). It’s a general purpose space for entertaining so, I can’t put up walls or anything. Not sure if room treatment would be effective?

Mine is a kind of irregular shape with cut corner on the rear wall, simplified size is about 19 x 19.3 x 10.7 ... Room treatment is a very effective tool to improve the quality.

In your room you have to find the right placement of your sub and the whole setup for optimal playback, especially if you want to improve the bass. So the bass traps is the first thing to start with and it will not cost you much. Also absorbing panels on first reflection points (walls and ceiling). Some companies can offer free room analysis (or for a very modest cost).

If you never did it with one sub and 2 channel playback then don’t add another sub!

If you have a home theater setup in this room then it is even more problematic.

If you want to stick with the integrated. Then add a sub… same brand and model. Two will extend the soundstage. The only trick with them is to not turn them up too much or the pinch the center soundstage. Also carefully integrating them. 

I adore my 1200 Mk5. Unfortunately, I have little experience with its stock arm as I have never used it!

You definitely need to play to the arms strengths and choose a cartridge that allows optimal synergy. Numbers don’t always tell the story and I would give more credit to actual users.

FWIW, you might consider a suitable headshell spacer since the stock arm height adjuster does not always allow the tonearm to go low enough for optimal VTA with many cartridges. https://www.analogueseduction.net/headshell-leads-cartridge-fitting-essentials/oyaide-mcs-cf-head-sh...

In a 15x30x9 room to use only one subwoofer is never going to be optimal, to put it as mildly as I can.  One very good reason for using two or even an array of subwoofers is to cancel standing waves.  You would be well advised to read the information available on the Audiokinesis website and even to talk to Duke, the proprietor.  Assuming you want better low bass definition and extension.