Does an upgrade make sense?

It is the reason why i never go on with sub after one trial to do so...

It is way more complex that it is presented by sellers and costly if we want to do it right...

I use another road...

I improved my small active box speakers with a rear porthole with a bundle of TUNED (different lenght and diameter) of straws put inside it...

I go from 80 hertz in specs to 50... With no downside and a very clear punch bass.. No boominess..

Each speakers is an Helmholtz resonators then we can take advantage of this...No sellers of speakers will sell them with this advice or with tubes going outside of them like a decapitated bouquet of flowers ... 😊

With some rearrangement of furnishing etc in the room you may be able to create more of a long side and a short side. Place speakers so they fire along the long side of the room. Let the speakers breathe - have space on all sides, and work with exact toe-in and positioning. Next, consider a turntable and phono preamp. If you do need subs, consider four of them, just for the lowest bass, with less crossover problems (e g Audiokinesis Swarm). Personally I gave up on subs after some years with a Velodyne DD18, replacing two REL Strata, and others. I like better bass but not the solid-state and equalized type of sound.

You already know this but the sound stage depth will be limited by how far your speakers are from the wall.  Since you can't move them further away from the front wall  I suggest removing the TV and filling the space with a diffuser.  This will effectively "move the wall back" a bit.  Also, swapping the glass front pictures with something non-reflective will also help.  IMO this is the best upgrade you can make at this point.  Adding better/more expensive gear will have the benefits compromised by the room not being optimized.

I totally agree with the addition of a pair of subs.  Bass traps really help in a two channel system.  My soundstage became much cleaner after adding mine.  Good luck and cheers.

Chord DAC's are quite good and there are a few at various prices that should be fine for your budget range. 

Since you can't move them further away from the front wall  I suggest removing the TV and filling the space with a diffuser.  This will effectively "move the wall back" a bit.

If this isn’t possible you should at least throw a blanket over the TV when listening that’ll also help quite a bit and very simple/easy to do.