Any cables that give more bass

What is wrong with running your speaker wires thru a crossover, prior to the signal getting to your mains? That's what a number of subs have integrated in....they can either take an RCA direct input, if your pre has sub outputs, or you can connect your speaker wires and then run your speaker wires from a different set of terminals (crossover hi output). Sunfire subs do this...many others also.

What is wrong with running your speaker wires thru a crossover, prior to the signal getting to your mains?

From a purist perspective, the last thing anyone wants to do is route the signal (ultimately destined to the main speakers) through a subwoofer's cheap high-pass network, internal cabling, switching, connectors, etc.

The OP's integrated amp has one set of pre (line) RCA outputs which is perfect to drive a powered subwoofer.
Just let the main speakers rolloff naturally as they do this quite high already....and integrate a sub with one IC cable to add some presence to the bottom 2 octaves. It does not get any simpler than this.

Djtiti, The problem with using a loudspeaker with more bass extention in a room as small as yours is there is no way to control the bass. If you use a subwoofer you can control the level with a knob on the back. You can't do that with a full range loudspeaker unless it has built in subwoofers with eq and level control. Ideally you could get a sub with a eq built in for even more control. Jafox is right about using the pre out of the DK to the sub. I've done this before and it works just fine. I think a sub is the best way to go in that room.

Djtiti,
Matching the speaker’s output to a room can be as important as choosing the speaker itself. The interaction of room and speaker is crucial and often overlooked. Over time I’ve learned that a small-sized speaker in a small- to mid-sized room (like my own) can sound like a very large speaker in a very large room. On the other hand, a large speaker in a small room often sounds like.. ahem...crap most of the time. That is the reason why I am an advocate of small speakers+subwoofer in a smallish listening environment.

Placement of floorstanding speakers, either towers or bookshelf speakers on stands close to wall boundaries will often make the bass boomy, voices chesty and midrange colored. Imaging will suffer too. Although bass is stronger where the speaker is closer to the wall, more so towards the corner, the latter is the worst location for all other frequencies and this sort of situation has led to the popularity of subwoofers. They can be located where they produce best-sounding bass while smaller speakers can be located where they deliver the best sound and imaging for the bulk of the audible spectrum. Since subwoofers operate only at low frequencies of typically below 80Hz, they are very tolerant of where they are mounted as the wavelength radiates in all directions.

That brings in the question to the size of your main speakers, the KEF Ref 203. I've looked up that speaker and feel it's a relatively large speaker given the size of your room, and as said earlier was a little surprised that it didn't go lower than 55Hz given the size of the drivers and cabinet enclosure. If you really like the sound of your KEFs(as you have suggested in your last post), I would suggest keeping the speakers and getting a subwoofer to reinforce the bass if it's only the bass that did not live up to your expectations. It may be your DK Design amp, or it may be cables(as some have suggested) but if I were you I would follow the majority of forummers that have advised on getting a sub. You can end up burning more money on cables, unless you don't mind having a few pairs for evaluation purposes later. Good luck.