Paul Smith called me sometime over 20 years ago and relayed the Carlos Santana's complaint that when he was playing with a wireless, the guitar was too trebly. I suggested that it was created by using a short cord instead of a long jack cord. Well, having a long cord was not too workable, even wound up, so I suggested using a delay line. After messing around with a bunch of lines and investigating the market, Paul settled on a 75 ohm, 150 nanosecond, 10 section delay line and then put it on a double pole double throw mini toggle switch so it could be switched in and out.
In retrospect and considering the work I have done since, it probably can be done with a resistor-capacitor network as well - rather like in the high-end cables.