Any cables that give more bass

I would also look at a solution other than playing with cables.

The KEF 203 is rated down to 55hz. I would run a sub no higher than 60hz or so. The natural rolloff of the KEF's would make it unnecessary to insert a highpass filter before the amp to drive the KEFs.

.....you wire your left and right amp outputs thru the sub and then the crossover in the sub feeds the non supb output to your speakers.

There's too much to be lost by running your signal through a subwoofer that then will drive the main amp to the main speakers. I would try every other option before this.

Does your integrated amp have line output and power amp input connections? Can you use these and still retain the link between the internal pre and power amp sections? If so, you could run a line level IC to the powered sub and be done with it. This would allow the KEY to run in its current configuration.

With some integrated amps the pre/power link is broken if a cable is plugged into the line out or main amp in. If so, you could use a Y-adaptor off one of the line outputs, either L or R, and drive an IC jumper back to that amp input and run a longer IC to the powered sub's line input.

With the "right" level setting on the sub, this could be a very nice result. I did a similar task for a friend who has simple Polk speakers and the result with the sub was quite impressive.

Many self-powered subwoofers connect via the main speakers connections or via the amplifiers output terminals so having a sub output on the amp is irrelevant.

This is fine, but I would NEVER use a signal coming off a subwoofer that would ultimately drive my main speakers. And that was the point of my post before.

With a speaker that truncates the bottom 1.5 octaves, no cabling, equalization or speaker positioning is going to help with this. It's either a subwoofer or another pair of speakers altogether.

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.