After extensive testing, I chose Vovox for my KH o300 speaker system.
When I added the Neumann sub a year ago, I was in a hurry and used 2 different cables to hook it up to my Apollo. I totally forgot to exchange them and only noticed whilst tidying the cable-spaghetti behind my desk.
So I ordered a pair of Vovox which arrived at midday the following day (excellent service by Thomann btw). I exchanged the cables and was gobsmacked.
The subwoofer has a much tighter low-end. The mid-range is much clearer than before and the stereo image has improved dramatically.
I do additionally use Reference 4 but this is something that everyone should fix. I'd say it's improved my speakers' accuracy by approx. 15% as they are being fed a better signal. I am extremely pleased and it's added to the enjoyment of my work.
The price is pretty steep but if it's your profession, I'd highly recommend!