Dear Worf,
Since I never posted before and putting the cart before the horse, I preface my remarks before doing my profile. As a full time clarinetist, etc., I supported myself working in an audio shop Glen Head L.I. This experience 5 decades ago, had me meet and test gear for the likes of Sol Marantz and many others.
General comments first:
It maybe comforting to keep the familiar, perpetuating the "they don't make things the way they used to" mantra. Unfortunately as your friend knows, most truisms, and many laws are not only misleading, untrue, counter-productive, etc. but no longer serve their original purposes. I restore things like Fairchild turntables, but pay dearly to solve hum and rumble issues.
The receiver must first be isolated, as each pies in his chain needs this connection in another setup. Then if his speaker cabinet is especially good, replace all cones as you intimated. Use one of the 3 or more suppliers that list details electrical, acoustical, and size s/a Madisound. Replace, do not clean the old cables unless they are very special gold laced latnum $50,000. greats from the future. q tips are great for many things, not cables and tape heads. Alcohol has water, and along with the cotton swabs leave debris, making matters worse. Use cleaners meant for this application.