Sometimes, ignorance is bliss
If you are happy with your setup, then there is no need to change. I know there are some people in this thread who are recommending subwoofers + DSP, but I have to warn you that subs may not be easy to integrate into your system and may involve a bit of a learning curve.
What I am going to suggest will not cost you any money, and you can do it right now. I understand that you have placed your bookshelf speakers on your study desk with your monitor in between, correct? And the rear of the speakers are fairly close to the wall? You could try these:
1. Fix your bass by positioning your speaker. Any bass that comes out the rear port of your KEF LSX II will be reflected by the wall and interfere with the front firing bass. It may interfere constructively, in which case you might get more bass. Or it might interfere destructively, where you will get less. The interference will be frequency dependent, and this will manifest itself as "one note" bass where some bass notes are louder than others, or complete lack of some bass notes. Use this
online tone generator and play some bass notes (i.e. 20 - 120Hz). Position your speakers by pulling them away from the wall until most of the notes sound the same. If you don't want to use your ears, then download an SPL meter or pink noise analyzer for your phone. It will tell you what the problem frequencies are, at your listening position. Then use a
free equalizer for Windows and remove the loud notes.
2. Elevate your speakers. Speakers placed directly on the desk will have "desk bounce" where sound reflections from the desk interfere with the main sound from the drivers. This is often out of phase with the main sound and can cause
comb filtering. You also ideally want your speakers at ear level. Place some books under your speakers and see if it improves the sound. If it works, and if you don't like the look of books under your speaker, you could (1) wrap some bricks in paper and use that as a stand, (2) construct your own stand, (3) buy stands.
3. Eliminate desk resonance. Speakers placed on a desk will cause the desk to resonate at its resonant frequency, which can colour the sound. If you can feel your desk vibrating while you are playing music, then it is resonating. You could mass load the desk by placing heavy objects on it until it goes away, or you could decouple your speakers by placing something absorbent under them, like rubber foam, a folded up towel, etc.
4. Engage pressure mode. Try to seal your room and make it as airtight as possible. Close the windows and the door, stuff clothes under your door sill, etc. This will cause bass to behave in pressure mode (or room gain) which will increase bass. The better your seal, the more effective this will be.
Of course, the better way to do it is to go down the route that others have mentioned: i.e. buy a microphone, get a MiniDSP, buy subwoofers. This is the best route, but it will cost you money.