Solid brass is pretty simple to polish; there are household polishes sold that usually have some abrasive in them (like Brasso) that will remove tarnish and gunk. But that company was right; anything like that will damage the thin brass plating on the pipes. You can go ahead anyway and then have the pipes re-plated. You can use non-abrasive tarnish removers (which generally require dipping the parts in a solution and soaking a while). Or you can do what sculptors and antique dealers do and refer to the "patina" as something valuable that shouldn't be removed.