I have SE535 and have no issues with muddy sound but I don't use the stock cable, custom silver cable. You can go for better iems with better drivers and amount but it sure will set you up in price. If you still want to keep your SE535 that's the next step to improving them else get your cash out and spend more on say seinheiser IE800 that apparently are perfect stock AK100 buddy, not sure about the RWAK100 though maybe someone here can comment. If you read prior posts many hifi-head experience listeners have mentioned their gears.
I have a null-audio lune series cable with my se535 and I am looking to sell it....I have just ordered a fitear F111...hopefully I will receive it soon :-)....I will also look for a replacement cable if all goes well with the F111.