Could be you’re ready for some new strings? Just because they don’t break is no reason not to change them once in a while. They do lose sound quality.
Here are some possibilities.
An article on strings suggested removing them, wrapping them around a coke bottle, and then reinstalling them. It’s supposed to knock out some of the grime. No idea how well it works.
Use tarnish-resistant strings and colored C & F basses. I don’t, based on the opinion of my harp technician and, I think, of Saul Davis Zlatkovski here on the Harp Column. (Saul, am I right? You did say the old-fashioned non-tarnish-resistant strings sound better?)
What NOT to do: don’t use copper polish or anything like that. Save that for your samovar!