I have to agree that the cables are the most likely candidate. That includes to a lesser degree the speaker cables as well. What H/K separates do you have? Even the cable between the preamp and amp will have an effect.
Digital components also tend to pollute the AC. If you don't have a power conditioner; or one that segregates the digital from the rest of the system, then it is a factor also.
Digital components also tend to pollute the AC. If you don't have a power conditioner; or one that segregates the digital from the rest of the system, then it is a factor also.
The bottom line is, maybe it not one thing, it is a number of small things compounded.
It is hard to convince someone who may not believe cables matter. I do not know if this applies to you. Think of it as installing budget passenger car tires on a sports car. It just won't handle the same and the high end tires.