Whoever told you to "upgrade the cable" is a fool, and unfortunately they make up the vast majority of the "audiophile" market. There are a number of products that have integrated PEQ that will solve this problem for you. Qudelix if you're looking for a low cost solution, and miniDSP is out there too. Some higher end DAPs have PEQ built in, some higher end desktop DAC/amps have them too (Fiio K19, RME ADI-2 DAC FS, etc) and I think there are a few other portable DAC/amps with this ability too (Fiio KA17, Neutron HiFi DAC v1, the Qudelix 5k or T71, etc). You can also employ software DSP implementations using tools like the Hang Loose Convolver which is truly the ultimate DSP in terms of absolute performance.