In certain situations a servo ic is used to eliminate coupling caps from one section of a given circuit, utilizing the ic in a feedback loop, usually at the input. Although coupling caps are a necessity in tube products, they do have their own set of coloration that add to the sound of the product. A tone control of sorts. Some designers felt that by removing them at the input of the circuit and replacing them with a servo ic there would be less coloration. What we now know is there's no free lunch. Servo ic's do have their own set of coloration with each manufacturers ic's sounding different. In some cases they can actually sound worse than a high quality coupling cap.