Tubes age. The presentation softens some when they are 'old' and they can get noisy. The best thing you can do to avoid angst over slow degredation is to have a spare set on hand and, depending on amount of use, put the new set in every year or so and see if there is any meaningful difference. If not take them out and want another year or so.
You can get tubes identical (brand) to the ones you have, or if you want to change the tone you can consider other brands.
You can get tubes identical (brand) to the ones you have, or if you want to change the tone you can consider other brands.