Typically tubes slowly deteriorate… exactly as you describe. I would replace them and keep your existing ones as backups. I always have a set of backups anyway.
I always try and buy my repacements from Audio Research… both because they extensively test them and to help support a great company. But you can save money by buying from The Tube Store, Upscale Audio, or other. Make sure you get matched sets. Most of these places sells “replacement” sets for specific components.
I have all tube gear… all ARC for my main system. See my UserID.