I have some general comments, but they will not cover the entire scope of potential issues with your old amplifier.
First, replacing old capacitors with new capacitors will make a huge impact in coupling circuits. This is the very best "boon" in hifi I have encountered. Good coupling caps (sonicaps for me) have a monumental impact for very few $ invested.
Second, all electrolytic capacitors have a fluid internally. Over time this fluid will dry and the etching material becomes frail. Electrolytic capacitors will "age" with time. Given the age of your amplifier, all of your electrolytic capacitors should probably be replaced. Fortunately, there probably aren't many of them in the old Quad amp.
Third, while amps are not my area of expertise, there is a guy that knows this stuff. Sheldon Stokes
http://www.quadesl.com/schematics.shtml is a major bucket-o-brains on this material. He remains fairly busy, so don't expect a quick response to an email. However, his response will be honest & informed. I suggest you email sheldon with your query. I am certain Sheldon will know something about your issues.
While most folks aren't "running 1801s with hamsters"
they probably don't know much about your specific application. Quad made some good gear back-in-the-day that is used quite often. I am sure there is hope for your old amp. I am not certain you will find the answer here.
